Have her see a Dr ASAP, sounds like BV or yeast infection.
Or Trich .. Trich has a very foul odor especially after her period. Also she may have a thick white discharge, itching, accompanied with a foamy or bubbly discharge. I know this may seem gross but if you notice a foamy or mousse texture ring around your penis after intercouse she has Trich ... I can tell the difference between a yeast smell and STD smell, there's a very distinctive stench. A FOUL odor is a sign of an STD, whereas a MUSKY smell is usually yeast ... Hope that helps, you should get tested yourself and make sure your tested for Trich coz men usually aren't ... Then you'll know for sure. Don't wait for her go YOURSELF!!!!