What Causes Painful Boils With Pus On Vagina And Thighs During Periods?

I get painful boils before and after periods.the skin aroubd gets hard and hurts .at time it burst opens up with pus..it majorly happens during and around periods time.not sure why I keep getting them it simetimes comes on thighs..I have seen doctor and got tested for std and other infection but all tests was negative