When a guys heart beats superfast what does it mean?

The other night I was chilling with my friends with benefits we were cuddling. While my head was on his chest I could hear/feel his heart beating REALLY hard and fast, I mean like as if the kid had just finished doing jumpingjacks for a half hour! We hadn't done anything yet sexual wise so its not like he overexerted himself (unless one can do so by lying comfortably in bed!). We have never officially dated although we have some pretty complex history over 5 years so its not like I am some new woman that he would get all excited about finally touching him ''down there'' been there done that plenty of times! My question is did he feel something that made him a little scared that he is more into me than he thought or would like to think he is? Or was he just really horny?Do guys hearts beat fast when they get an erection? Also is it possible for guys to try and convince themselves they don't care and they are just ''f*ing'' because, they are scared to be vulnerable? (Espescially with someone who was once young and has stupidly turned them down in the past ahem..me?)
When a guys heart beats superfast what does it mean?
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