Pain in upper left arm, also in armpit. It hurts to move it behind my back, like to reach around to grab something, very weak. Pain is unbearable?

See below. This could be related to your shoulder or even a pinched nerve in your neck. Pain in the upper outer arm with difficulty reaching behind sounds like your rotator cuff. This is usually treated with anti-inflammatories, rest, ice, and gentle exercise. If pain lasts greater than 2 weeks see your physician for further workup.