I have suffered this numerous times, at the same tracks you have too, German long track and Cadwell, it happens very quickly.
What usually happens is I start to get a nauseous feeling at the back of my throat and higher stomach, and if I carry on I am positive I will be sick, because I usually stop.
Obviously doing things like tracks days are a tense experience, and I was very nervous/apprehensive. So was not eating or drinking well.
I did though do about 10 laps of the long Snetterton and was fine, the feeling was there, but nowhere hear as bad. So I can only put it down to incline, although it seems to be corners that bring it on, and it got worse over the years. I did take a nausea pill there though, well half a one.
I play simulators a lot and have never experienced it there, but have always struggled with things like waltzers, fast carousels. but am fine with roller coasters and things going up and down.
I did also have nausea in karts, EVERY SINGLE time I drive one, I would do maybe 3 or 4 races in a corporate thing and then start to feel it.
never been able to read for long in cars, and had ear issues as a kid which I am sure this is all about. Damn annoying but I do think I would be fine doing something like oval racing or hillclimbs, minimum time in the car. Also am fine on a motorcycle.
It is motion sickness, brought on by the inner ear not being able to link the inputs your eyes are seeing and your gut saying "I tell you what can fix this, I will make you sick" Ar singly annoying, utterly frustrating. And unpleasant because the nausea can last for hours afterwards.