Left arm pain radiating from back into arm, sometimes wrist and palm. Hurts to stretch and lift also. Pain is 2-3 mild. Osteosarcoma? Pinched nerve?

Cervical HNP. Arm pain in young patients radiating from the back or neck, suggests disc herniation. This is most likely. Osteosarcoma is most common around the knee, humerus, pelvis and jaw. This is usually associated with a swelling, and likely night pain. Very rare! if you see hoof prints, think horses not zebras. Osteosarcoma is a definite zebra.
Nerve. Unless you have a history of bone cancer (osteosarcoma), this sounds more like a pinched nerve in your neck.