For the past few days after having anal sex, my anus has been sore and irritated. At first it was an itch, but then after about 2 days it developed into a sore pain that is intensified when I pass waste. It has been very red and swollen, and there are two small red/pimple-like bumps on the outside. The sex was unprotected, so I have a feeling that this was the main cause, but my partner says that he was clean. What could it be and how can I treat it?