Dull ache in left forearm above and under elbow. pain comes and goes but gets worse when wearing tight long sleeves. any ideas what it could be?

I would suggest that you check your arm for any changes like in the skin or compare it with the right arm to see if different in anyway like swelling. It may be a neuropathic pain due to pressure on a nerve that may be entrapped at the elbow. There is not enough information other than give you a general idea and consider physical evaluation by a clinician.