Why does my boyfriend behave so unfriendly?

I have this guy who used to be so close and shared everything with me when we used to be friends. But now when we are in real relationship why does he try to go far or he doesn`t care about me. He used to text me from morning to night about 100 times a day but now even I text him he replies so slow and just about 5 or 10 msg per day. What`s wrong? Do all boys behave like this after getting in relationship?