What Causes Swelling And Pain Of Vagina?

Hi Dr.,

I am 24 and from the past few months i m experiencing the following problem :
the area between vaginal opening and inner portion of right vaginal lip gets swelled up and it also pains..can you please tell me what is it?
it happens at times and then it becomes fine...
i have been sexually involved with my partner for over an year but for the last 2.5months we haven't had sex.
can you please advise as to why it swells?

thanks
bb