Arm rash, doesn't itch, isn't bumpy?

i have a rash that developed on my arms a few days ago. it is from the top of my hands up to my elbows. it's only on the undersides of my arms. it isn't anywhere else on my body. the spots aren't raised and they don't itch. they're brownish in color and about the size of a pinpoint. i thought it might be heat rash, but it's been 3 days now. any ideas?