Arigatou vs. sumimasen

Read your own writing.
Then a bunch of teachers in the kitchen looked a little shocked like I'd just said something really bad.

But you are still focusing on cooking and waiting rather than the situation you caused others to be shocked and corrected you.

Only you can find out by asking him or others why they were shocked.

I already asked them why they were offended! The English speaker said 'sumimasen' was the wrong word to use in this context but didn't elaborate on why and what the context was in their eyes. And when I paraphrased him and said 'So when someone's doing me a favour it is rude to say sumimasen?' he responded with yes but didn't elaborate on why it was 'rude'.

I told my supervisor about this situation because I'm very upset that I may have offended others by using the wrong Japanese phrase or not understanding what was going on around me. I explained to her exactly what I explained to everyone here, and she just said 'So you used the wrong phrase, so what?'. But she didn't explain why the phrase was wrong, and she didn't ask anyone involved in the situation what caused them to be offended by me.

What else am I supposed to do? I was hoping my supervisor would ask those involved to get clarification on what exactly happened but she didn't. She's very busy and doesn't have time for this.

Don't throw a temper tantrum like a little girl by saying I don't want to speak Japanese anymore.

Well would YOU want to use Japanese anymore after this incident? Everyone got offended with me and no one involved will tell me WHY so I don't know how to rectify it. Furthermore, some people on this forum have told me that using sumimasen, a phrase I was taught is polite and standard, is 'annoying' and 'inappropriate'. I'm SCARED to talk to people now because if I can't use 'sumimasen' properly without pissing off other people, how do I know I won't make more mistakes and cause more offense using more complicated phrases? It's not about throwing a tantrum, it's about worrying that I will encounter more situations like the one last week, and upset more of my coworkers without realizing it.

"sumimasen" is not an almighty word.
the word has at least two different meanings, as stated by others here. that will make a confusion or a new trouble.
I use "sumimasen", only in the cases of "Excuse me, may I ... ?"
otherwise, the word means your apology. it does not have the meaning of "thank you".
I know some people use "sumimasen" a lot. but, generally speaking, it is not appropriate in most cases.
I think it is the time to start to increase your vocabularies.

"sumimasen" is not an almighty word.
the word has at least two different meanings, as stated by others here. that will make a confusion or a new trouble.
I use "sumimasen", only in the cases of "Excuse me, may I ... ?"
otherwise, the word means your apology. it does not have the meaning of "thank you".
I know some people use "sumimasen" a lot. but, generally speaking, it is not appropriate in most cases.
I think it is the time to start to increase your vocabularies.

I know it doesn't mean "thank you". I intended it as an apology. What I'm asking is why using "sumimasen" and "gommenasai" were rude, as I was taught "sumimasen" is the way to say sorry for inconveniencing someone else. And yes, you are 100% right that I need to increase my vocabulary. I'm trying to do this the best I can, but learning a new language doesn't happen overnight. Currently I'm trying my best with the limited vocabulary I have, but it's very discouraging when I use a word that I think is right for a particular context and it turns out to offend everyone. Maybe the problem wasn't the apology at all, but that they misunderstood what I was apologizing for as ".." said.

Anyway thanks everyone for your input.

So, this person says a BUNCH of things you absolutely don't understand. Then, you respond to this by apologizing and then bowing a lot. Why not just tell the person you didn't understand the entire message? And ask to hear it again? Or hear it more simply? Well anyways, one likely scenario is that you apologizing and bowing a lot came across as sarcastic or a jab towards their culture, since maybe doing those things was not correct in that situation given the words your colleague may have said afterwords. I know I'd be annoyed if I said a bunch of things completely unrelated to something deserving an apology, and the other person just started saying sorry and bowing a lot.

Well, with only knowing only a small fraction of your colleague's message, it's impossible to tell why or why not your reaction was incorrect. The best we can do is guess.

So, this person says a BUNCH of things you absolutely don't understand. Then, you respond to this by apologizing and then bowing a lot. Why not just tell the person you didn't understand the entire message? And ask to hear it again? Or hear it more simply? Well anyways, one likely scenario is that you apologizing and bowing a lot came across as sarcastic or a jab towards their culture, since maybe doing those things was not correct in that situation given the words your colleague may have said afterwords. I know I'd be annoyed if I said a bunch of things completely unrelated to something deserving an apology, and the other person just started saying sorry and bowing a lot.

My colleagues already know I can't understand 90% of what they say to me. I tell them many times "wakari masen" or "mo ichido onegaishimasu" but they still talk to me in Japanese because they don't know any English. I could understand the gist of what he was saying - that he's next in line to use the microwave, which is why I apologized, because to me it was obvious that I was holding him up. I had already started saying "sumimasen" before he said the other stuff that I didn't understand. Like you said, maybe he was saying something else after that and I apologized inappropriately. This particular teacher works at a desk close to mine so he knows that my Japanese level is really low from interactions we've had in the past, and simply from observing my daily struggles with basic tasks like answering the phone. He never makes small talk with me, he only talks to me when it's about something practical eg: wanting to use the photocopier or microwave. Whatever the situation was, I feel really ashamed that my poor Japanese ability has caused such a misunderstanding in the office, and feel really stupid and dumb. Obviously I need to keep studying in order to prevent future misunderstandings, but in the mean time I feel really scared to talk to my colleagues in case of making a mistake and causing more people to be offended.

Just to clarify, when you had asked the English speaker the first time, did you simply ask: was sumimasen the wrong word to use "in this situation" -- or -- did you ask more specifically: was sumimasen the wrong word to use "when you wanted to apologize for holding the teacher up from using the microwave"? There must have been some misunderstanding about what exactly you were wanting to apologize/say sumimasen for since they expected you to say thank you.

Can you go back to the English speaker (the one who said sumimasen was the wrong word to use but didn't elaborate on why it was rude) and tell him that this has been troubling you for some time now and that you want to better understand. And so, can he elaborate more on why it was wrong and why it was rude?And ask him, if they expected you to say arigatou, can he elaborate for you what exactly you should have been thanking the teacher for? This English speaker might be the best person to elaborate on the situation since he was physically there to observe what happened and what was said. And in addition he would be able to speak English to you so that you can understand.

A lot of misunderstandings can come about when people make assumptions or presume the other person knows what they are talking about, so be sure that both you and the English speaker are very specific when talking with each other to clarify this misunderstanding.