I honestly cannot answer why this is occurring, but I can encourage you to set up an appointment with your healthcare provider; yeast infections are the absolute worst and this must be so hard for you to deal with on top of your period. Trust me -- I waste no time dialing up my healthcare provider when an issue like that arises! Just know you are not alone, most women have experienced this. Just reach out and get some help.
First, are you sure they are yeast infections? If you have not been to your healthcare provider for a culture to confirm your diagnosis, do it. If indeed you’re getting yeast infections consistently, something is up with your vaginal flora. Hormonal changes associated with your cycle can change vaginal PH, creating an environment for yeast to grow. After all, the yeast is there anyway, simply growing out of whack for one reason or another. That said, let’s address a few simple preventative measures. Always wipe front to back when urinating or after a bowel movement. Always wear cotton crotch underwear, doesn’t matter how pretty (or not) the rest is, but the part that touches your vagina should be cotton. Anything that creates a warm moist environment will increase your chances of infection. Change shortly after exercise, watch the tight jeans, and say goodbye to thong underwear (for now anyway). Underwear should always go in the dryer to kill any yeast left there, simply washing and hanging will not do. Eat a yogurt a day or start taking a refrigerated supplement called lactobacillus acidophilus. This is the active culture in yogurt that promotes good vaginal chemistry.
This is not intended to be medical advice. Everybody is different so please make sure to consult your physician if you're having issues. Do not delay or refrain from seeking professional medical advice from your physician because of something you have read on this site.