My DS2 has this at the moment (or something very like it).

He got a mozzie bite which he scratched and somehow a staph infection has got it. It started small but is now about 4 cm long and looks raw and weepy.

I went to the Dr and they cleaned it with some liquid and covered it (which DS took off as soon as he came home!). I have got some strong antibiotics for him as the infection is a tough one. He has to take the medication 3 times a day on an empty stomach.

Go to your GP ASAP as the infection will keep growing if it is the same one and take longer to heal.
pretty mcuh exactly what i was going to say, dd had this, on new years eve we took her to the dr, he said a mozzie bite had become infected wtih a staph infection - she was put on antibiotics, we used bactraban cream, and she is still having anti-itch baths.