That's your anterior ligament telling you if you keep doing what you're doing, it will tear.
First, that type of stretch should be done after a workout when your muscles are warm, not before. Even then, if you're trying to improve your range, you should be letting yourself down slowly with your arms to resist body weight and gravity. Pressure should be constant. Don't bounce. The worst thing you can do is try to rush it. Be careful. It'll come.