I find that most kids go through phases of increased motion sickness around that age. At one point, I remember paying 100$ 4 times in one month to get my car professionally cleaned after vomit incidents in the back seat. Put the seat in the middle if possible, adjust the angle of the rear-facing seat to a more upright position (within seat parameters, of course), make sure the straps are well-adjusted and that the kid is not wearing anything near their neck, teach them to look in front of them, not through the side windows, play around with music settings (some kids feel better with music, others without). Other than that, try to drive as smoothly as possible until this phase is over and, if possible, try having someone else in the back to supervise.
If you just read that and thought "rear-facing?", please look up why it is crucial to keep kids rear-facing as long as possible, but at the very least until they turn 2.