Im having upper arm pain that sometimes goes down in to my forearm. my upper arm is the most hurt part. i sleep on my left side sometimes & sometimes for too long. if i kept my arm bent wearing a sling, will that help whatever it is heal?

This is not a simple answer with the brevity of information. To identify cause we would need additional information such as ... was there an injury; what makes it better and worse; are there other symptoms such as weakness, numbness, tingling, etc.; do you only have it when you sleep?

If it is not an injury a sling is not an ideal treatment. If the hand goes numb it may be a pinched nerve in the neck or thoracic outlet syndrome.