Stress and maybe the temps were too hot or too cold for those 3 hours she was in the car?

Having the guy take it in a cheezit box wasn't the best thing either. She was probably scared and nervous.

Another thing could be when she got to the new home, he didn't properly take care of her but that is less likely as you say she died in 1 day.

She could have had parasites you didn't know of as well. Just throwing some things out there.