What Causes Heat Rashes Which Raises On Scratching?

I have what starts out like a heat rash, and when I scratch it it becomes raised. I used calamine lotion and tea tree oil, and it keeps it flat and stops the itch, thus stops further irritation as well, until I miss a time. If I continue to scratch, it turns red and then scars dark brown. It is on my shoulder, above my clavicle and almost halfway acrossed my chest just below my clavicle bone on one side only. What is this?