"Good" pain versus "Bad" pain

There's nothing absolute about having to have your body and arms parallel to the ground. And you don't want to feel this in your lower back at all. Definitely sounds like you are going too far and also that your form isn't quite right. Try bending your knees a little, externally rotating your legs so your pelvis has some place between your legs to "nest," and only go to a place where you feel the stretch in your hamstrings (in the belly of the muscle, not behind your knees) in a bittersweet way (pleasure and pain mixed perhaps - not just pain). your back should still have its midline groove and not be rounded at all. If you want to accelerate this process, take a look at the book "Relax Into Stretch" and follow his guidelines. You will need a trainer / partner to help with the passive stretching, which is very effective.