friction sore or ?

i am a 28 year old female, overweight and at least twice a month (usually around my period) i get what seems like either a boil/pimple/pus and blood filled sore on my underwear line on my inner butt cheek close to my inner thigh.  i dont know if its caused from friction from tight underwear and wearing a pad during my period or if it could possibly be something else.  i do have stretch marks there too.  it is quite embarrasing.  its not near my vulva or anus, but i am still concerned it could be soemthing as far as an std.  does this sound like an std?  what should i do to alleviate them from coming back?