It sounds like you were bit by a spider or a mosquito. With any bit if it gets infected it will get pus in it. You need to use alcohol or peroxide to clean it, then put a anti-bactrial cream on it like neosporin. Do that 2 times a day and it should clear up. If you are experiencing a lot of pain, and its not clearing up in a day or 2 call a Dr. I hope that helps.
