I think this might be something you need to desensitise to, by perhaps going out and sitting in the shade for a while. That way you can become more used to the heat.
I used to have the same problem, and I would advise that for a short time, you cut down HEAVILY on your dairy intake, paricularly milk and chocolate... When I had the problem, the doctor made me remove them from my diet briefly, and it actually solved the problem. Turned out I was being irritated by something in the milk, and the sun was catalysing that irritation.
Hope it helps you like it did me.