Sex is a very complicated issue. there could be many reasons he doesnt want sex and none of them have anything to do with you. It could be stress from exams , although sex is a stress reliever. He needs to talk with you about what is really going on. You can' t be understabding if you don' t know what it is you are supposed to understand. It's not fair to you for him to simply refuse you
physical attention with no explanation.