left shoulder pain

[ydob]

here's my situation, whenever I bench or do BB shoulder press, I experience very sharp pain in my left shoulder and left shoulder only. The thing is, I only feel it when I do certain weights. ie. for benching I can do 135lbs or less all day but when I put 135lbs or more, I can barely do any. Same goes for BB shoulder press, I can do 95lbs or less all day but when I put 95lbs or more, I can't do a lot. I'm not new to lifting weights, been doing it for a year, just frustrated that my bench and shoulder press aren't going anywhere.
I've looked at the RC injury thread and tried the exercises and I also looked at thoracic mobility stretches but the pain is still there, anyone know what the problem could be?