have you been concentrating more on exams than him? or has the topic of exams come up more often than others?
right now you need to turn all your upset into positive energy for revision!
Easier to say than to be done i know, but try not to let this take over right now.Who knows what will happen after these exam, you may get back together, but you may not.
He may have had reasons like feeling not so wanted or something, but f not then is he worth pining over if he is able to break up with you at a time like this..
Good luck with your exams! (i too am going through a break up type something and doing A-levels so definatly can sympathise)