It sounds like a cyst. I also think you should get it looked at. They are not as uncommon as one might think. If it is a cyst, and you find that they keep re-occurring, hormones may be the root cause.
I'm not entirely sure what it could by but by the sounds of things, if I were you, I would go get that checked because it doesn't sound common.
It could be something as minor as a cyst, but I definitely recommend getting it looked at by a doctor.