This is Dr. David
it sounds like you severely strained or tore a muscle below your arm pit, most likely your pectoralis major or minor muscle
you need to ice the area with bags of ice wrapped in a towel and alternate with heating pads to help relax the muscles.
I would not use TENs units on this area.
make sure you take advil with food in your stomach, never on an empty stomach and also with plenty of water since advil can tax your kidneys.
you should see your doctor if the area doesn't get better
let me know if you have other questions.
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