Talk to Dermatologist on Heat Rash

I have been suffering from this allergy since three years back.When I feel warm in summer red patches with small white pimple without puss appear on my full body with hard rash from head to toe.It happens even in winter,when I feel warm in my blanket during sleep.I visited some doctors but as I did not take medicine it happens again.I want to doctor to test my blood and take my skin part and check it through microscope like 'skin biopsy'.So that I can come to know what is going on inside me.And the last thing I have to tell is that it is not some food allergy or dust allergy.I have completely change my diet but it remains.