Hurt my back

I was following the stronglifts program yesterday and I hurt my back again. Here is how it happened. As I was squating, I felt a "movement" in my lower back. It was an odd motion, and it must have been me rolling my hips. When I squated 85 pounds I did not have this trouble, but for some reason with 95 pounds I did. I think this may be a flexibility issue and I will work on my squat stretch and leg swings daily to loosen the hips. I will also keep my weight at 85 pounds for the next week to build up and work on my squat form. I will keep all my weight the same for one week