It might be chicken pox give her some children's benadryle,I would cll the dr asap because chicken pox for a infant is or can be dangerous.make sure you give her plenty of water or pedilite that is the best.this will give her the electrilites that she needs to keep her hydratded. Best of luck!
Heat rash usually shows up on the body where the heat is the most intense, like the neck, back, backs of the knees under or around the arms. I think you should take her temperature, and go from there, since she has them all over the body, I'm leaning toward the chicken pox, especially if she is trying to scratch them. Take her to the doctor. Hope this helps, good luck.