Vulnerability

Why is it that after sex with my boyfriend I get so vulnerable? The last time (about three weeks ago) I was butt naked in tears and he was holding me really tight. I don't know what it was that made me cry but it happened and I was kind of embarrassed. He kept asking me "did I do something wrong?" And I felt so bad because I didn't have an explanation. Has this happened to anyone else? What was his reaction?