You are correct. In particular, the first has a specific time at which you should be doing your homework (now) while the second entirely lacks a time frame. Even the implied "before it is due" is weak, as the homework in question could already be overdue.
So, sentence 1 says you are doing something other than your homework, and whatever that is, it is not what you should be doing at the moment. Sentence 2 says you are not doing your homework at the moment, but at sometime you should do so. This leaves open the possibility that what you are doing now is what you should be doing at the moment.
Note that neither implies that you will do your homework, only that in your opinion you should do so.