I have a red lumpy rash on my neck and chest. Some areas are tiny whiteheads. Does anyone know what it is?

The doctor has told me it is prickly heat. But I don't often sit in the sun and haven't got particularly sweaty recently. Anti-histamines don't seem to make a difference. The 'spotty rash' takes about 5 days to go. I wonder if it could be stress related or maybe something to do with moisturising my chest when the weather is hot