Yep, after. Or after a massage. Don't want to do a static stretch on a cold muscle, and post w/o static stretches are great for recovery and reducing DOMS. Lower back is a trickier area to stretch, but if you play around with some stretches by "feel" you should be able to find one or two that work well. Might be able to get some ideas by googling too.
update, I started drinking a gallon of water everyday and my lower back is fine. Im also taking milk thistle but I was taking it the whole cycle, the water seemed to really help it. Cant forget how important it is