Why do i hate him after ?

i have a wonderful boyfriend and we have such a great relationship and life. the only problem is after we've had he suddenly repulses me, i go quiet and don't want to look at him. he's starting to know somethings up the only problem is i dont know what. i love him so much but for some reason after we do it i become moody and irritable and i just don't want him near me!! anyone know why this is????