My lower labia minor has got a pink blister which pains to touch.this happens 2-3times in a year. and after giving hot fomentation either it subsides or sometimes a pus sort of liquid comes and after that i feel better.. what could be the reason ?

Here are some...: Has it occur at the same spot and is there a tiny lumpy spot after healing? If at the same spot with no residual node after healing in 2-3 weks, it would be a case of genital herpes, which may be eased if taking antiviral drugs withing 24 hrs feeling itchy/sensitive at the spot; otherwise, just keep the area as clean & dry as possible. In fact, you have handled it well over the years.