My great Nan sometimes throws up in the car and at home out of the blue. Why?

She may have an intestinal blockage, but more likely I would think she has eaten too much and it just hasn't had time to begin digestion. Try smaller, more frequent meals and see if that helps. Also try to keep her sitting upright for about 30 minutes after eating.

Could it be her digestive system not working properly? She always bumps andnher tummy is like a hard ball sometimes

I suggest having your relative see her primary doctor to see if she gets motion sickness while riding in a car, and/or if she has acid reflux, both of which could make one toss up. I have both issues myself, but Dramamine over-the-counter med helps with the motion sickness.... and I try to watch my diet with the acid reflux but some times it can still tackle me :(