Your hamstring stretch probably sucks too.
I know because I have done http://www.exrx.net/Stretches/Hamstrings/Seated.html forever, and I got royally bitched out by my PT yesterday for it.
The problem that I had - and that the chick in the link has - is that I was rounding my lower back which removes the lower back curve (lordosis) and that puts extra stress on your lower spine. This is especially bad if you already have low back issues.
The solution is to sit down, exaggerate the lordosis, get your legs out flat in front of you and then tilt forward from the pelvis. Since the hammy attaches to pelvis, this ends up a much more effective way to stretch. I could feel the tightness immediately.
Same deal with standing hammy streches - keep the low back curved.