Extreme nausea after af ended?

AF started the night before Christmas eve, it was light. On Christmas eve it was still light. On Christmas Day it was EXTREMELY heavy. So heavy that I thought I was gonna have to go to hospital or something (bled through super plus tampons in 20 mins) but throughout the day it returned to a normal like flow. And then the rest of my period was normal & ended normally. AF ended around Thursday-Friday. I had unprotected sex twice yesterday and ever since lastnight I've been suffering from bad nausea. No vomiting, but just out of no where nausea. Smells have been throwing me off & I don't really know what to eat. The nausea gets worse at night. I also suffer from car sickness and it feels like I've been riding in a car for hours. My stomach just feels empty & weak. Thinking about the wrong foods or smells could send vomit out of my mouth any second. Why sudden nausea? There hasn't been a change in anything except for me having sex lastnight but I didn't have sex today and the nausea is even worse tonight.

*This was also the only month that I had PMS symptoms. I used to get really bad PMS symptoms every month before AF but I randomly stopped getting them 3 months ago. So this is the first time in 3 months that I've gotten PMS symptoms (sore boobs, mood swings, cravings)*