Your probly damaging your rotator muscles. They are the tiny muscles all around your shoulder joint that are normally not used in the manor of pulling (in your case with your left shoulder "resisting") 50-55 lbs. You should probly lay off shooting for a week or two and then see if it still troubles you.

I started shooting my bow last winter and I was also working out pretty hard. I damaged some rotator muscles and to this day I cannot bench press without major pain. After running about a mile my shoulder is inflamed with pain. I went through rehab but it didnt help much. It sucks, I can shoot without pain but I cant excersize like I used to, and I am only 24 years old. So take 2 weeks or so off and see how it is from there.