Men are like this!
Women are emotional beings and they are insecure that they are in constant fear that their boyfriend will start to lose interest in them after sleeping with them. Which is quite natural. Men are like this! But no one knows the reason, why?

Expectation
As a human, you would obviously expect to bond more with your partner after taking your relationship to another level. You want them to spend more time with you, give you more importance and if your partner is showing the least interest in you, then you should second guess your decision to be in that kind of relationship!

Tell him, "It is Okay!"
Women mostly ponder upon, that they did something wrong and that it is their fault. They blame themselves for repelling their boyfriend because of which he eventually gets worried about you being worried. What women don't think through is that he might have been going through some of his own insecurities. If you are worried during the act, that will definitely get him worried and this won't work in your favor at all! Maybe, he wants you to tell him that it is alright between you two and that nothing is wrong!

Not Serious!
One more reason behind his disinterest in you is probably because he was never really serious about you at all. Men do this all the time! He just wanted the act to happen and now he is just looking for excuses to end the things between you two!

Commitment issues
He must have commitment issues and it is quite normal for men to feel this right between their relationship. It makes him worried and the realization might have kicked in that his freedom might be at risk or maybe he now has a responsibility. Or maybe he must have developed a crush on someone else and that your relationship with each other has nothing interesting in it anymore.