Almost Peed On Him

My boyfriend and I have sex a lot, but the other day, we had sex once and he finished but I didn't. So a few minutes later, we had sex a second time once he had time to "recharge". When we were having sex, I had an incredibly strong urge to pee like I was going to piss with him in me. I told him to stop and he did, and as soon as he pulled out the feeling was gone. Why is this?