great, thanks for the advice
I'd have it looked at if I were you, just to rule out anything serious. It's always best to put your mind at ease with a professional opinion if you're at all concerned.
I highly doubt it's an STD. It could be many a number of things.
I had something that sounds exactly like what you described. To this day I still have no idea whatsoever what it was, but I went to a doctor and found out that it is definately NOT an STD.
Note: If you do go to a doctor, do not call it a "sore". I described mine as a sore and they immediately thought herpes. I was scared to death, but the blood tests and cultures came back all negative for herpes. You'd be alot safer calling it "a lump under your skin" (since that's actually what it is, herpes is ON the skin, not IN the skin). I wish I had, it would have saved me a massive headache... Doctor said after all the tests came back that it was probably just a boil.