Did that really just happen to my boyfriend? LOL embarring NOT sure what it was boys help!?

So a couple days ago I was sitting in my boyfriends lap well not in his lap but like his legs were spread and I was between you know? Well anyways we were sitting there talking and flirting and then he said we better go back inside (we were at a little kids basketball game at my school in his car, but no one could see us cuz that would have been embarring!) So whenever I got up out of his lap he was like oh gosh (or something) and his face kinda turned red and he kinda like pushed on his pants and but his hands in his bball shorts like he was trying to push something down. But at first I thought I like hurt his so I was like oh sorry did I hurt you or something??! But he didn't really answer, so what I'm wondering is... did he really get a b*ner? lol and why was he sooo embarred we've been going out for about 4 months and were both 17 so I would think he wouldn't be that "ashamed" for lack of a better word. IDK?!