My boyfriend cried yesterday?

Yesterday, my boyfriend and I were making out on my bed and were getting pretty into it, then all of a sudden he slowed down and started to cry, which I've never seen him do. I asked him what the matter was, and he just said he was "stressed by his grades in school". I didn't believe that at all (he is super smart), and later he said that he was overwhelmed by all the and things like that. He is my first boyfriend, and I'm his first girlfriend, we're both 17, and we've been together for 9 months. I just don't know if he was overwhelmed, or felt that I was pressuring him. He is a sensitive guy that respects me - any thoughts as to what made him cry?