Why is my penis swollen after sex? No pain, but swelling wont go down 8 hrs later and there's discoloration as well; no itching either.

Erection. Are you talking about the erection or size of the penis in width. Are you taking any medications that could cause sustained erections or a medical problem ie sickle cell anemia. Check it out.
Priapism or edema. Priapism is an erection which remains hard for hours. This needs to be treated, hence all viagra (sildenafil) & Cialis advertisements to call your dr. Or go to er if erection lasts more than 4 hours. Longer the erection lasts beyond 4 hours greater risk of permanent damage. Soft swelling of penile skin can result from prolonged or rough sex & will settle down on its own.