I'm a few years older than you, so here's my take on your question. "Fishy" tends to mean imine molecules, the source of which can be some form of BV, or bacterial vaginosis. They used to call this "gardnerella", but now acknowledge that it is a non-specific form of bacterial overgrowth in a woman's vagina causing havoc.
Yeast infections are something alltogether different, caused by a candida albicans (normally harmless fungus). This tends to create more discharge of the cottage cheese consistency. I still don't buy the "stale semen" theory, though many on here have tried in vain to convince me.
My vag odour (healthy, and verified normal) has always been pretty noticeable all the time, esp musky after intercourse, and even pre-puberty. Even with semen inside for awhile, mine hasn't ever been remotely fishy. I don't care for it, but my man absolutely adores it. Keeps me in cunnilingus every day!! lol
See your gyn if it gets bad. Take care!
If you have BV, or suspect it, see your doc and get it checked.
I recently went to the dr because I had the same "fishy" odor. I had been having the smell for about 3 or 4 months. I thought it was a yeast infection & called the nurse who prescribed Diflucan. I took it & thought everything was ok. Then the odor came back. I finally made an appointment & saw the nurse practioner. She took a swab & looked at it, but didn't see any "bad" cells. But she ended up prescribing some internal vaginal gel for 5 days to see if it helped. I just ended the prescription & so far it seems to have helped. I didn't have any of the discharge, just the smell. The nurse practioner said it could be lubricants, birth control, hormonal changes or just the mixing of his semenal fluid with the my fluid. But if it continues I would check with your dr to make sure everything is fine.