Hmm, that's odd. What kind of pain?
Mine is so hard in the morning and if I awake at night that it kinda hurts, not painful, but it is so hard that it is a little uncomfortable, I rarely jack it off when it is like that, but I love to have sex with it like that as my stamina is insane.
It is never that hard at any other time though and never uncomfortable any other time.
Is this what you mean?