Babe..Go To Your Gynocologist.
It could be Anything from a Blocked Gland,to Herpes,an Infected Hair Folicle.....the list goes on.
Putting off Finding Out is gonna make it Worse on You. Don't be Embaressed..Docs have Seen and Heard it All.It's Better to Just have a Pro Diagnose You than to get yourself All Psyched Up about what it Could Be.
In The MeanTime, Try a Hot Water Compress On It **if you can Bear It** If it's A Blocked Folicle it May Make it Go Away on it's Own by Helping it Drain **Hence the Hot Compress***. Do NOT TRY AND POP IT!!!!
Take Care of Yourself Hon, and Good Luck... Hope it Something Simple.