To add to what @Markinzy said, it is important to warm up your body before any strenuous physical activity. This includes stretching. Think of your tendons as strips of plastic. When plastic is cold, it is brittle and can snap more easily. When warmed up though, it can be quite malleable. Furthermore, you have to trust yourself, if you're doing a stretch and is feeling really painful, reassess. The pain doesn't always mean you're just not flexible, it can also mean that you're pushing your body past its limits. So just keep in mind to listen to your body.