You Could Have an STD
Common STDs are known to make your vagina smell funny from time to time. Gonorrhea, trichomoniasis, and chlamydia might make it smell, er, fishy down there, and you may have some discoloured discharge simultaneously happening.
The first step is to speak to your doctor and get tested. If you do have one of these common STDs, there's some simple medication that can take care of it, so nothing to worry about.