I too got dizzy, felt sick to my stomach, etc.
first thing; don't eat before class
second; try focusing ahead of you when you start and finish the roll, that helped me.
third; make sure you are breathing
One thing I find helpful is to do a back roll, and immediately follow with a front roll along the same line (they take the same line diagonally across the back) both forward and back. This can help make too.
Hope that any of these can help.