I need help/advice on why I feel this way

Me and my boyfriend have been together for a couple years now and I love him to death, we live together and I don't see myself with anyone but him we have a great relationship and we're really great together. For some reason today though when we went to have sex I just sat there the whole time while he did his thing and I wanted to cry. I have no idea why though it was great but I just felt so sad and I have been a lot lately...