ithappened, this may or may not help, but when I get lower back pain I lower myself into a deep squat and hang there for a little while (30 seconds or so). I do a few reps of this and it really helps loosen all the joints, ligaments, etc. Obviously, some prenatal yoga might help too, but I've found the squats give immediate relief.
I've also been hanging out in frog pose after workouts (sort of like child's pose but with your legs out at 90 degree angles like a frog). It's a great hip opener.
You might also want to try stretching the ol' hamstrings, since those are one of the common culprits of lower back pain. It's probably a combination of things, since your back is under added strain your other muscles are having to compensate and therefore tighten and you've got all the whacky ligament stretching and tightening. I was having really obnoxious back pain for a few weeks there but the combo of my new pregnancy pillow and religiously stretching my hips (frog pose and squats) has eliminated it. And I have a really whacky back thanks to bulging discs.