Y after unprotected sex my foreskin is swollen?

Assuming it is not: From a prior std, the microtrauma to tissue during sex can cause edema(swelling) and foreskin is well vascularized. This is why it is so easy to transmit std's, as the skin is damaged and fluids wash in and out of the micro-tears, the std's are easily transmitted. That being said, recommend protection! hope that answer helps.