Crying after sex

Charlotte
A few times after my long term boyfriend and I have had sex I have cried a little bit. Tonight, however, I couldnt get a hold of myself and full out sobbed. I felt completely great and happy emotionally and mentally, and am so happy to always be with my boyfriend and I love him so much. So I'm confused as to why I seem to cry sometimes. 
Does anyone else have this happen? Does anyone know why this happens?