I had a backache last week so bad I literally had to crawl up the stairs to get to my bedroom. Oddly enough, two acetaminophen (which have always been on the list of acceptable drugs to take during pregnancy; I'm not one to flip over taking any OTC medications) knocked it out enough for me to be able to move again without wanting to cry, I even managed to stand up long enough to make my husband lunch for the next day. Didn't kill the pain completely, but I know better than to expect that.
Since then I've taken acetaminophen twice when I felt a bad backache coming on, and it's gotten rid of it both times. Not sure if it would help you, but it probably won't hurt to try.
You can also try a hot shower. Works best if you've got a handheld sprayer & can aim the water exactly where you need it. My bathtub just isn't deep enough or shaped well enough for a hot bath to work the same way for me, but that's another suggestion.
If you don't want to take any medicine, the ThermaCare patches also are very good in my experience. They're charcoal. Unless you're very thin, get the patches made for menstrual cramps instead of the ones for back pain; they've got adhesive so you can stick them to your shirt or pants. It's another of those things that you have to be really prompt about, though, because they take forever to heat up & don't work til they're warm.
You might want to check out this page on Optimum Fetal Positioning
(basically the same information as at Spinning Babies, but more helpful to me. It might well be a muscle-tiredness issue rather than a fetal-positioning one, from the sound of it, which means you're stuck with treating the symptoms rather than the cause.