Being that a cup of coffee is 80-120 mg looks like you're taking about 4 cups of coffee which in my personal opinion is a high amount of caffeine to ingest daily.
Caffeine has massive diuretic properties so if you aren't getting enough water this could lead to dehydration and possible rashes, I had heat rash before from too much diuretic once because my skin dried out and being out in the sun made me have a rash. Not saying that's what yours is but make sure your water uptake is sufficient and honestly is cut back the caffeine addiction to anything to the point you feel you need it is never good. Not bashing just saying.
Just my .02 on the caffeine