Armpit or underarm pain can be caused by a number of things that include pinched nerve, pregnancy, cancer, swollen armpits and lymph nodes, muscle twitch, breastfeeding, waxing, cystic pimple, cancer, among other causes.but in your case there may be- Brachial plexus nerve injuries - Shooting pain Pinched nerves especially brachial plexus from stretching, pressure or trauma can lead to this problem. This is often accompanied by numbness and tingling feeling and pain that resemble electric pulses. But exact diagnosis is made on basis of investigation or examination.