It sounds like BV to me, which does not need to come from a partner, so you can go to your mom with a clear conscience. (Besides, by your own account, it was there before you began having sex.) Do see a doc and get treated, though, as it is something that can cause you long-term problems if you were to ignore it. Good luck, hon. Get some protection and on the pill while you are there.
The bad thing is that in such situations it is never advisable to get something from the store, coz you don't know what you have exactly to know how to treat it and moreover if you don't treat it on time or exactly how it should be treated, things can turn bad for your reproductive system. There are tho some tests in the stores to check if it is fungy or bacteria but I have no idea for their names and I'm in a different country. The best approach is to visit a gynecologist. This is your safest bet and well your mother shouldn't know you make sex. Yellow discharge and a fishy odor is a sign of either BV or an STD, so check with a doc. Hope it helped. And read in google for BV and STD to find more.