only been studying two months - why negative?

Dear All - I have been studying Japanese for a few months now. I recently went to Japan and told people I am a student of Japanese (in Japanese). I asked someone how to add "but i am only studying Japanese for two months". I was told to say the following:
でも2かげつしか勉強していません。 I understand completely the sentence except why use the negative form of studying? Shouldn't it be でも2かげつしか勉強しています since I am currently studying . To me it seems like I am saying but I am Not studying.
Thank you so much.

I am sorry, but you don’t understand the sentence.
You have here,
…しか…ない construction. It means …only in English.

This construction is not a pure negation, but used to enphasize limitations, in your case, a very short period of studying.

If you want to say I have been studying for only 2 months, you'd say ”勉強(を)始めてから未だ（mada）２か月です。
でも2かげつしか勉強していません　is your reply to a comment/question that you know very little Japanese and not meeting one's expectation.
For example, if you are asked you have $10, you say but you only have $2, でも$2しかありません。

thank you both so much for answering this question. although i am new to Japanese I now understand completely. Looking forward to learning the subject more in depth with my teacher!

That reason is "謙遜(ken-son)" not same "$2" case.
http://ejje.weblio.jp/content/%E8%AC%99%E9%81%9C

So you can say all.
・日本語の勉強を始めて(から)、もう/既に 2か月(2ヶ月)です。 : positve
・日本語の勉強を始めて(から)、 2か月(2ヶ月)です。: standard
・日本語の勉強を始めて(から)、まだ/たった(の)/僅か2か月(2ヶ月)です。: negative
But Japanese using negaitve phrase usual except for family/friends.
BC "Pride and Bravado is not a Virtue" as their culture/principle.
(「自慢」や「虚勢」は美徳ではない。)

Maybe this is better explanation with a lot of sample phrases.
http://bit.ly/2eyV1Hu

しか always takes a negative.
It's basically the same as saying "I haven't been studying but two months." That "but" takes a negative "have not" but still means you've studied two months and are still studying.

Just remember しか as "have not but" and だけ as "only" and you'll remember that the former takes a negative and the latter a positive.