Could be a localised oedema (fluid) or a haemotoma (blood clot). Was there any trauma preceding the lump ie rough sex, or even sex/masturbation in general?
Go and see your doctor.
no.
you did not give a description of it. and anways no one here is a doctor and you need to see a doctor if your that worried about it. it could be a number of things...but if its in your vagina...you should be at the doctors