What Causes Red And Swollen Foreskin After Sex?

Hello,my boyfriend and I have been together for a year now. We have unprotected sex, as I'm on the pill and we been checked out before we we stopped using condoms. At the moment, we're having a long-distance relationship and can only see each other once a month. Occasionally (so far it happend 3 times within the past four five months) after sex my boyfriend's foreskin becomes red and swollen. He tells me that it's painful when he pulls it back. Usually after a couple of days of abstinence he's fine again. What's wrong? Please can you help?