If his feelings changed after you had sex then he may have gotten scared and ran, if things were fine before you were intimate he may have an issue with intimacy and showing his feelings. It could be many reason why he has this issue, if as a child his parents weren't good at showing affection and saying how they felt this could be a learned behavior that he got from his family or he could have had bad experiences in relationships in the past, it may have been that after he was intimate with his past his relationships may have changed after that. He has to be willing to talk to you and open up about what is bothering him and what happened to change.
Communication is key in a relationship and unless you can get him to open up and talk to you then the relationship is not going to last much longer because you are the type of person that can talk about your feelings whereas he is not, it comes easy for you but is harder for him, communication is the best way to know where the problem lies. Communication gap worsens the relationship. Unless and until you talk over about the problem you will never be able to get to the root of the issues. You have to take the initial step to come up to him and talk about where the relationship went wrong. When you share your opinion and feelings with him it develops a greater understanding. It helps him understand what you are feeling and what you need out of the relationship. So you do not end up in a fight but come out with some kind of a resolution.
Talk to him and see if he will open up to you enough to tell you what is wrong and what happened from the time your were intimate until now and why he feels the need to cease all contact with you. If he cares about you he will give you the answers to the questions you have and if he does not care I would move on to someone that will respect and care about what you feel and even though you have feelings for him he doesn't care enough to make you feel important and special in his life.
Good luck and let me know if you every need my help again.