I experienced the exact same problem once, the only difference being that I was too stupid to stop going heavy (for me anyway).
After the ripping sound and sensation from my lower back area occured - don't let this happen, it's not as much fun as it sounds - I went to a physiotherapist and was told that there was an imbalance between my hamstrings, glutes, lower back and abs.
They were acting against one another, so whilst I though I was getting stronger, I was in effect just making it easier to injure myself further down the line.
I'd recommend that you get this checked out ASAP, and work on your own weak areas. Dave "skinny arse" Tate has written extensively on this sort of thing, only usually without recommending a serious injury first
Hope this helps