I don't know how this could have happened, but the only time I've had these symptoms was when I was doing martial arts and someone would mash one of my ribs. All this, particularly the pain in breathing, is a sign of rib pain. It could also be partly an oblique strain, as it seems to be going up your side. My guess is, this is a strain that will pass from overstretching. Pretty impressive feat, though.
It could be any of the following! nerves, bones, muscles, tendons, ligaments or joints. Underarm pain is often the result of traumatic or overuse injuries. Underarm pain can be mild, moderate or severe, depending on the cause of the pain and the extent and the type of tissues affected.
The brachial plexus is a network of nerves that travel from the neck through the axilla or armpit to the upper extremities. Brachial plexus injuries occur when these nerves are stretched or torn, which may happen when the shoulder is forcibly pushed downward while the head is pushed up and away from that shoulder.Common symptoms include pain or discomfort under the arm, arm numbness and weakness on the affected side, and decreased motor control in the arm or hand.
You didn't say how long ago this happened. Have you used heat and ice?
If it's just the same then see your Dr.