Aw, poor thing. Car sickness is horrid. My kids are older and I don't know anything about the types of car seat you mention but, from experience, I would hazard a guess that she has a propensity to car sickness that's been triggered by not being able to see out so well.
My DD started with it at around the same age when she was in her own FF car seat but in a different car (hire car on holiday) that had much lower seats than our car at home. Looking back on it, her view out was definitely restricted by that.
Once we got home she didn't have another incident of sickness until she was 4. Then we were in our own car, own car seat and no obvious trigger. From that point it became a regular occurence.
She is a teen now but still suffers with it, mostly on very twisty/turny roads - she will be absolutely fine on a 5-hr straight drive up the motorway but even driving round the industrial estate 5 mins from home can set her off - and it is always much worse if she can't see out. She is almost 100% better in the front passenger seat with a clear view ahead.
Bit of a long-winded response but I would definitely try swapping back to the other seat if only to see if it makes a difference. Any chance you could borrow a FF seat in the right weight range that would fit in your current car and see if that helps? If it does, you'll maybe have to buy a different car seat, but at least it's cheaper than a new car!
It's so hard when they're little and can't articulate how they feel. DD rarely actually voms any more but the nausea makes her feel dreadful. A tell-tale sign for her is excessive yawning. That's always been a warning for us that we need to crack open the windows and swap seats if necessary. And even now we still often resort to I-spy type games to get her looking out of the window!
Hope you're able to resolve it at least for another few years.