What Causes A Bump Under The Clitoris?

I was masturbating and experienced two orgasms, still feeling arouse I continued to 'explore' and found a small bump under the hood of my clitoris near the back, at the base. It is sore to the touch. This has happened once before also, I've noticed that if I cause further irritation the small bump enlarges. Can anyone explain this please? I find the feeling uncomfortable and the situation a little alarming.