New to this board, but I have a couple that are by far my favorite for my back. Child's Pose is the most relaxing for me, I also really enjoy the stretch in Downward Dog. If you have a stability ball you can try the Swiss Ball Superman. That one will work your back muscles, but it feels amazing.
Get a foam roller! I had really bad lower back pain for years after having DD#1. I recently started using my foam roller on my back and the pain is nearly gone. It's mainly a runner's stretching tool, but it does wonders on your back.
Thanks for the suggestions and links! As it turns out, the pain was the first signs of an ovarian cyst rearing its painful head, so I'm down for the count right now. These will be good once the swelling goes down, though
Re: Stretches for back pain?
I would suggest the cat cow stretch
https://yoga.about.com/od/yogasequences/ss/catcow.htm
Here are some additional poses for back pain - I am not sure how flexible you are, so you could look at these and pick ones suitable for you...GL!
https://www.yogajournal.com/poses/finder/anatomical_focus/lower_back
https://www.yogajournal.com/poses/finder/anatomical_focus/upper_back