I know what you mean! I googled it and found, and I quote,
"The cum contains enzymes that interact with the mucosal lining of the vagina and the uterus in order to facilitate conception. It's an inherent trait of semen, and may affect other mucosal linings, as in your stomach. This might cause your stomach to get upset".
It should normally go away after half an hour or so. Can help if you have food in your stomach too to help settle your stomach :/ don't know if I helped but good luck n ting. :)
Drink some liquid after you swallow and it will help, my girlfriend says that it always feels OK.