Men and women react differently to orgasm, especially strong ones. And men usually have strong ones, whereas women have all kinds -- often in a very short time. It leaves men flowing with the body's natural opiate-like substances, and makes them very tired for a while. Spent. Whereas a woman will often get the opposite because testosterone levels are very high upon orgasm, and will have tons of energy and want to go vacuum the house or take a jog. The truth is, men and women are different species.
I am not a doctor, but my husband seems to get like this also. To me it, he just seems tired and drained like he let everything out and doesn't have anything else to give.