Hello, You are having sharp stabbing pain on the underside of your right arm between elbow and armpit. No injury, no numbness, no tingling but your arms feel weak. Armpit pain commonly occurs due to infection, injury, muscle strain, enlarged axillary lymph nodes, shingles, nerve compression, etc. This is concerning as nerve damage due to injury or compression may cause radiating pain in the neck or in the brachial plexus (a cluster of nerves in the armpit). You are suggested to get clinically examined by a GP, get MRI (neck) and EMG of the arms done to see which nerve is affected. You may be referred to a neurologist for detailed study on nerve damage that is causing you symptoms. Get nerve conduction velocity test and other tests done. Get treated at the earliest. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Nupur K., General & Family Physician