Tightening up during sex?

Was having sex with my boyfriend and he stopped and said, “are you tightening yourself up?” I responded, “No, why?” To which he responded, “ oh okay, because it feels like my penis is suffocating.” is that a good or bad thing? He kept going but he wasn’t moaning or anything ( our first time) he was just breathing heavy... Also what causes guys to go soft during sex? Like everything is great then all of a sudden that happens?