I had a heat rash due to stress - I'm in College and we just got over a three-week-long strike, which means I have three weeks of homework to complete in the four weeks we had left, along with the homework that was originally planned. My Mom told me she used to get the same heat rash when she was doing exams in school.
After weeks of heat rashes coming and going on my arms, back and legs, I started taking sulfer pellets. From what I know, they are availible in Canada in Goodness Me! stores (where I got mine) and Fortinos (which also has excellent sushi). They are working well and I'm talking vitamin B to stop my poor stressed nerves.
I haven't been this rash-free in a while. If you think you have heat rash but it's not due to humid weather, it's probably stress. Just take some Sulfer and vitamine B complex (no less than 50mg per pill) and take a load off your mind.