My lower back gets extremely tight/sore when doing deadlifts, stiff-legged deads, and sometimes squats. The weird thing is that I only feel the stiffness/tightness for up to a few minutes after doing these exercises. For example, today I did 5 sets of squats (to parallel), followed by 4 sets of SLDL (lower back was too tight to do set#5). By the time I was done with shoulder work for the day, my back felt fine. This was around noon, and now at 10pm, I still don't feel any pain in my back. I don't want to hurt myself...does this sound like a weak lower back, or poor form...or is this just normal?