The best way to find out what it is for sure is to go to the doctor and get it looked at. From the way you're describing, it's possible it could just be an ingrown hair. They're not painful at first, but they can be after a few days. Everyone gets one at some point. It's not an STD or anything, though.
this could be any number of things going on. going to be seen and getting a proper exam if in your best interest.
grace