bubu and Meg_S hit it right on. It's a combination of strengthing your core, along with stretching the lower back and hamstrings. The flexibility in the hamstrings is crucial. I would put more focus into that until it heals then add the stretching to your daily list of gym activities. Your "core" is both your lower back and your abs and they need to be strengthed simultaneously. If there is an imbalance, you'll have the problems you are describing.

HTH

Great info everyone!

Sporto