What Causes A Painful Blister Between The Rectum And Vagina?

A few days ago i felt an irritation and when i looked i noticed a large pimple type thing in the space between my rectum and vaginal area. Kind of to one side. Its painful if i touch it. It looks kind of pink with a white head. Ive been married for 28 years, same sexual partner so im pretty sure its not an std. I just noticed today right as u come up the line of the butt, below the lower back 3 small red bumps. I dont know if tight thong underwear could cause this or if it something more serious.