What Causes Recurring Hard Blister Near My Vagina?

I dont know why i always get a pimple like sore next to my vagina, very much like a boil but it never pops, and seems solid, and within a few days it goes away and always comes back days before the next period....why is this? It is also on the same side i seem to be irritated every time my husband and i have intercourse. Why??