i sort of get this, i wouldn't call it nausea, but it might be the same. The feeling is almost like an upset stomach or intestinal pressure in my lower abdomen. It can feel like a dull cramp or ache that feels warm, other times it makes my tummy sort of gurgly and spreads upward.
I never gave it much thought since i just guessed it was the combo of what and how much I ate, plus things inside me having to move and make space. Hasn't been severe enough to cause issues, but sometimes during sex I'll need a break or position swap.
Immediately after sex it usually gets worse, then fades away slowly. i feel normal or oddly light inside 6-8hrs after, and it's always worst when I am unaccustomed to the deep thrusting.
Hope that helps