The symptoms you describe are exactly those of an abscess.
If you get it looked at early enough, then you may be able to sort it out with a simple course of oral antibiotics.
However, the longer you leave it, the worse it will get, and it doesn't take long before antibiotics alone become useless. Then you need t get the whole thing lanced and drained, and it ain't pretty. More than that, you'll be kicking yourself for not just going to get it seen at your local needle exchange or drop in centre etc.
There isn't a 'treat it yourself' way of getting through abscesses. Plus, heating up the area with a hot compress, given the area is already clearly infected, will simply help spread the infection and bacteria.
Just go and get it seen to. The sooner the better!
's had abscesses before, and you do NOT want to leave them. If it's hot to the touch, swelling - basically doing everything you described, it's infected.
Hopefully you'll just be on a course of antibiotics. Good luck, and don't waste time!