I just remembered in Japanese

How do I say "I just remembered" in Japanese? I always need to use this phrase, but I don't know how to say it properly. 「さき覚えた」「今だけ覚えた」doesn't seem right to me, but I can't think of what would be right to use colloquially.

For example, my friend came over the other night and helped me with my homework, and I just now remembered that I didn't even offer him a drink or ask if he was hungry when I came over. How would I say, "I just remembered, when you came over the other night you said you were hungry, but I didn't even offer you a drink!" I'm thinking something like 「今だけ覚えた、部屋に来てくれた時君はお腹がすいたと言われたけど水もあげなくてしまった、ごめんなさいー！」

Another example would be, I told my friend I would meet her for lunch, but I just remembered that I have a meeting during lunch break. Would I used the same "I just remembered" as in the previous example?

One last example, I'm taking a test but I can't remember the material. Only after the test is over do I remember it. In this case, would I say something more along the lines of 「今だけ覚えてしまった」？

おぼえる is "remember" in the sense of putting something new in your memory, so "memorize."
おもいだす is "remember" in the sense of recalling (something from the past).

So in that context, the word to use would be おもいだす。

For the first two examples, I would say 「いま　おもいだした」(I just now recalled!)

For the last example, I would say 「やっと　おもいだした」= I've finally recalled! or「今になって　おもいだした」=I've recalled only as late as now (when it's too late)/It took me until now to recall.

We use " だけ " to express that there is no other thing, person or so of the same kind or under the same condition.
" 今だけ " simply means only 今 (= only at the present) and it usually does not imply the time being until the present.
So, you cannot use " だけ " to express "only as late as now" or "not until the present" or so.

If you recall or notice something only as late as now and you think you should have recalled or noticed it earlier, " 今ごろ " can be suitable. This phrase is considered to be abbreviation for " 今ごろになって " (literally meaning when the time goes around the present), and I suppose it can imply what "just" or "only" implies in your examples.

For the first example, you can say " 今ごろ気づいたんだけど、部屋で宿題を片づけるのを手伝ってくれた時、お茶も出さなくてごめん。 (I noticed it just now, but I’m sorry I didn’t even serve you with (a cup of) tea when you helped me finish a homework in my room)."

For the second example, I suppose " 今 " is enough for "just now" because you do not have to express that you regret you didn’t notice a fact in such a casual conversation with your friend (, unlike the case where your promise for meeting her is a business appointment). You can say " ごめん、今思い出したんだが、昼休みには会合 [kaigou] があって、君とは会えないんだ。 (Sorry, I recalled it just now, but I have a meeting during the lunch break, and I cannot meet you)."

Regarding some details, though off the question:

" お茶を出す " is a typical phrase to express serving a gusset with (soft) drink. We do neither use the verb " あげる " nor cite 水 (= water) as a representative of drink to express such serving.

Personally I avoid to mention that a guest asked for drink or food, because I think that in a common Japanese culture a host offers a guest small amount of drink (and food) as a natural way of hospitality, not only when the guest begs for it.

We use " しまった！" usually at the beginning of a sentence, like "Oops!" in English.

I wonder what you express by " 言われた " because here " れ " can be considered as either " れる " making an honorific compound verb or " れる " for passiveness. If you use " 言われる " for passiveness, you should use " 君に " or " 君から " here.

君 [kimi], though originally meaning a lord for a territory or a master of someone, is now in casual expressions of modern Japanese to refer to a fellow, a friend or so. In almost all cases where I express a person as " 君 " I do not use honorific words for the person’s deeds, even if I use some to express my humility.