Lately i've been getting a pain behind my upper left arm (like a dull pressure pain) whenever i sit. slouching makes it worse. possible cause?

Pinched nerve: Usually this is referred pain from the neck. Possibly a pinched nerve in the neck somewhere. Focus on slightly better posture and exercises the shoulders and neck. Let's see what that does.

Arm pain: This positional type of arm pain suggests nerve or nerve root compression from a bulging disc or spur in neck vertebra. A neurologist is best qualified to evaluate and advise you. Appropriate tests will be ordered.