M.R.
I don't want to alarm you, but it seems to me that the safest move now is to call a doctor. If you are in a hotel, I am sure they will have a person on call for hotel guests. If you are staying with friends or family ask them. Or go to a hospital.
You know, it could be anything. It could be fatigue, food poisoning, the flu. I am sure your child must be feeling tired and probably displaced.
We travel regularly between San Francisco and Italy, and I know well that if my daughter (now 3 1/5) doesn't get enough sleep on the plane, it can get ugly (grumpiness, refusal of food, sickness, you name it).
Good luck, and enjoy France!