What’s the difference between good pain and bad pain?

Good pain is a “comfortable level of discomfort.” This means going to a perceived end range within your body, without pushing beyond that edge. Generally our elastic connective tissues need some time and more heat to build up before they will yield to the deeper stretch and, ultimately, to a new end range. If you push beyond that edge too quickly, it is likely you may tear something. This may feel like searing heat or perhaps an internal electrical explosion. This is an important signal to stop what you are doing and immediately apply ice.