numbness on right upper arm what it could be?

Thoracic Outlet Syndrome

Thoracic outlet is the space between the collar bone (clavicle), first rib and corresponding ligaments through which nerves and vessels travel from the base of the neck toward the armpit. Thoracic outlet syndrome (TOS) results from a compression or extension of the subclavian artery or vein, or brachial plexus (nerves), commonly occurring in motorbike accidents, athletes, swimmers, weight lifters, etc. Symptoms include:

Muscle wasting at the base of the thumb, numbness, feeling of pins and needles, or pain in the shoulder, armpit, arm or hand (when nerves are compressed)
Pale, cool arm with weakened arterial pulse in the arm, numbness and pain (when vessels are compressed)

Radiation-Induced Brachial Plexopathy

Radiation-induced damage of the brachial plexus can follow radiotherapy of the chest, axillary region, thoracic outlet or neck. Symptoms may appear months to years after radiation therapy and include numbness, swelling, weakness or pain in the arm.
Broken Shoulder Blade

Shoulder blade (scapula) is the bone in the upper back that connects the collar bone (clavicle) and arm bone (humerus). Broken shoulder blade, usually from a car or motorbike accident, can result in pain, swelling, bruising or deformation of the shoulder blade area, and weakness, numbness or tingling in the shoulder or arm.

Broken Arm, Wrist, Hand or Finger

Symptoms of broken arm (the arm bone – humerus, elbow, and bones of the forearm – radius and ulna) include:

Severe pain increasing with arm movement
Obvious deformity, swelling, tenderness and bruising over the site of bone fracture
Stiffness or inability to move your arm, hand or finger
Weakness, numbness or tingling in the arm, hand or fingers

Cubital Tunnel Syndrome or Ulnar Neuropathy

The ulnar nerve arises from the brachial plexus in the neck and travels under the collar bone, downside along the inner side of the upper arm, behind the inner part of the elbow (Latin cubitus), where it can be felt as a “funny bone” and then down to the wrist, hand and little and ring finger. Ulnar nerve entrapment usually results from an elbow injury or constant pressure upon the elbow, like in cyclists or typists. Symptoms, known as cubital tunnel syndrome, include:

Pain on the inner side of the elbow or electric shock sensation after touching the elbow
The hand, ring and little finger are numb and falling asleep, especially after bending the elbow
Limited movements of the ring and little finger (“handlebar palsy” in cyclists)
Hand (on the little finger side) sensitivity to cold