Whether you are new to boxing training or you've merely upped the intensity of your workouts, shoulder soreness is a common problem. You can generally work through the aches, but it is important to not overdo it and invite an injury.
Punching puts stress on shoulder muscles, especially for newly initiated fighters. The less experienced or less conditioned you are, the quicker shoulder fatigue could set in. It may only feel like simple tired muscles, but if you overwork the shoulder you may cause something more severe that you really won't feel until the following day or two. What you are feeling is the stress of the anterior deltoid, the front shoulder muscle. This is a small muscle that typically doesn't get a lot of attention at the gym. But in boxing training you will discover it for sure!