I don't think this is hormones, but I really don't know. My therapist says that my phobia helps me not to force myself into doing things I don't want, and if I learn to listen to my heart and don't do things to satisfy others, I will not feel so sick all the time. If this only happends when you have sex with one specific person and it didn't happend in any earlier relationships, maybe you should question yourself if this person is the right one for you? maybe I'm totally wrong here, but it seems like n* is sort of an "alarm system" that we've created to help us get out of situations. If you think about it, that n* feeling has probably helped you get out of a lot of situations before?
I sometimes get this feeling, but I also get it in other situations that are satisfying in a way (like sex). When I go to a great music concert or do something I've wanted to do for a long time. It feels like my nerves are damaged in some way, as if the satisfaction and stimulation just becomes too much somehow. I'm also curios about what this might be. If it's something physical I'm thinking more about the nerve system, but what do I know? maybe it's hormones.