N.G.
DO NOT put a warm compress on what could be an insect bite. Warm compresses increase blood flow which can aid in spreading an infection. If anything, ice it, but do NOT put heat on it.
My daughter got a red ant bite last year that she had an allergic reaction to, which sounds like what you're describing. She had to get a steroid shot to clear it up. It was on her toe, and her whole foot swelled up.
If I were you, I would take a benadryl, and put hydrocortisone cream on the spot. If that doesn't clear it up, go to the doc for something stronger.