How can you tell if your baby has a rash from food or something vs a heat rash?

My son is 7 1/2 months old and he's had heat rash and broke out in a rash from amoxicillin. Now he has a rash on his back, legs, bottom of his feet and a little on his neck and face. He also has a fever, but he isn't fussy or craby. I thought maybe he's teething again, but he didn't get a rash last time, just a fever.

I would check with his ped over the phone just to be sure there are so many things out now you never know.GOOD LUCK