Pain is described as a ripping, searing, burning or pinching feeling in your body when practicing a yoga pose. If you experience any of these sensations stop the pose immediately and/or modify it so you are not in any pain. You will most likely feel actual pain if your body is out of alignment. You can ask your instructor after class to help you properly — and safely — practice the posture.
If you experience sensations of intense stretching but no actual pain, then you are in no danger. Make a distinction between the two and practice accordingly.