It sounds like you need to strengthen your abs, and entire core. When it isn't strong enough to hold you properly your lower back will burt. The reason I think this might be the case is because a) I used to have severe back pain when i first started working out and running and I did not work very much on abs or posture 2) I will get lower back probs similar to what you describe even now if I neglect my posture and "core" strengthening.

On top of whatever you do to strengthen it, throughout the day, and while running imagine lifting yourself out of your pelvis and hip sockets. Never run and allow yourself to "sag" into your body. If you can exchange your chairs that you normally sit in ie: computer, table etc. for an exercise ball that will also help out things in general. "core" strength and stability cannot be over emphasized.

Here is a good link: http://ourworld.compuserve.com/homepages/meriam/ Pilates and T-tapp are very good too.

good luck with the pain!

Meg