Left Arm Discomfort

I've been on a beta blocker (bystolic, 2.5mg daily) for about 6 months now. This was all due to a chest infection I had back in February that the doc thought sent me into atrial fibrillation. He thinks now it might have been a fluke due to the infection, but wants me to stay on the bystolic for a year to help keep my BP down. Anyway, that's just for background.
Since I've been on the bystolic pretty much, I've had discomfort that seems to center in my left arm. I don't have what most describe as angina in the chest area, mainly just the left arm. I do, however, seem to have chronic muscle pain in my left chest area. It seems to move around, and right now is centered beneath my left breast, feels like it might be around the ribs. But my question is about the discomfort...it worries me a bit, since I know a lot of left arm pain can be cardiac-related.
It's almost always a dull pain, not throbbing. But today, I did feel some sharp shooting pains right above my armpit area and into to the top of my left arm. That got me worried, thus the reason for my posting. I've monitored my BP closely for a while now, and nothing there is out of the ordinary. The pain is what is worrying me the most, aside from the occassional palpitation that takes my breath away (no pain involved there).
Please let me know your thoughts on this, and thanks in advance!