My daughter either has an allergic reaction to something, heat rash or viral rash.

My daughter either has an allergic reaction to something, heat rash or viral rash. She’s not been poorly with it, just really itchy. We have given her piriton which has helped the itching, but the rash is still there.
Question: if it’s a reaction to washing powder and we continue to use the same washing powder then will she continue to get a reaction?
The reason I ask is that she’s had this kind of rash before, but we never changed the powder and within a few days the rash cleared up?