I Think It's an Eczema ..
It might be keratosis pilaris. See:
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002433/
Have you touched anything recently or been near some plants?
If your arms have been not been covered up, it may well be an allergic reaction.
You do not say whether the rash itches or not.
If they are bothersome then see your doctor to help identify what this rash is. Without seeing your rash it is difficult to say what it is.
Wash the area with gentle soap and apply antiseptic cream.