I have been tanning a lot but also have been working out in the gym a lot, strenuously. i noticed a cluster of reddish purple dots on my side. not on the skin but underneath. looks almost like a blood vessel ruptured under my skin. what could this be from

Rash: This could be what we call "petechial rash" which could be from a variety of causes, from trauma to leukemia, so definitely get this examined. You didn't mention whether this location is one which might have had a recent trauma locally? I hate to hear about the tanning as it drastically increases your risk for deadly skin cancers. Fake tanners are the better way to go!