So I recently decided to get back into boxing and have run into a problem. After a few rounds on the heavy bag or even any drill involving keeping my hands up for an extended period of time I find my lead shoulder(left shoulder in this case) slowly starts hurting more and more. If I rest for a bit it continues and gradually dies off after like 5-10 minutes. I've thought of things like lack of stretching, weak rotator cuff, etc. Anyone else out here have a similar issue or know anyone who does? Any other ideas on how to prevent it? Thanks!