This is a very common injury, and you should probably change trainers because the one you have is either stupid or not paying attention. When you jab, you don't lift your left elbow. This breaks the line of the body (you aren't getting your shoulder and that slight turn of your hip into the punch) and puts all the strain of the punch on your rotator cuff. If the same dingbat has also convinced you that it is in your best interest to hit a heavy bag that weighs a ton, you'll tear your shoulder up real quick. If he also tells you to hit the bag as it swings into you, you are even more screwed.
If you are training guys to box, you have to do them the service of teaching them to punch properly, how to work a heavy bag, before you just cut them loose.
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