If it is pea-sized and does not move - I would definitely get it checked out.
Otherwise it could be muscle-related and eventually go away.
Check Dr Oz's website. LOADS of topics. Maybe he has an answer.
It could be a Bartholin's cyst. They sometimes drain if you soak in a hot tub. They sometimes go away by themselves too. If it gets too big and painful your only recourse is to have it treated by a doctor. Good luck.