Have you had this checked? I don't mean to worry you but this could be something serious. If the lump feels warm you need to go in to the doctor immediately. My daughter is only 16 months and she has frequent MRSA (type of staff infection) in that area and it can cause large hard bumps under the skin, IF there gets to be a head on it, do not squeeze it because it could push the infection back inside and it will come back again. I say you should go get it checked but I don't believe it's an STD!

Best of luck!