Flying...high on pain

I first experienced this with my girlfriend as she caned me, and later with the lash of a whip. It is difficult for me to get to the point where a particular part of my body is beaten enough to attain this sensation; the sensation of pain being a drug. In fact, I've only experienced it on three occasions in three years. Have you ever felt this? How often? What does it take to get you to the point of pain not only as pleasure, but as euphoria?