Rash on baby

[twinparent81]

Just wondering if anyone knows what this rash is on a 4 month old formula fed baby?

ah, seems i cant upload an image...! all over body rash, raised, red, disappears under the glass test

Could it be heat? With heating on/wearing knitwear? Like hives maybe!
My dd had a rash and turned out her super brand new skin was just sensitive to a top she was wearing but was very localised to her top half, not all over body.