How Can Sharp Pain Under The Left Arm Be Treated?

37 y/o, white, female. No history of heart or cancer problems. For the past couple of months, I ve had sharp, stinging pain under my left arm. It s terrible at night, if I roll onto my left side the pain wakes me up. During the day the pain is almost constant. The area is sore to the touch. There is no rash, change in skin color, fever, illness, bumps, lumps, or bruises. Ice does not help. I do weight lift, using 5-20 lbs, nothing outrageous. I thought it might be a pulled muscle, so I stopped lifting for a couple of weeks. That helped a little. But the pain is still there. Any suggestions?