I recommend seeing a chiropractor who practices Gonstead techniques. Ours is wonderful and we see him every week. My lower back and tail bone bother me a lot. I'm also at 31 weeks and really small too.
You could also try a brace. I put one on the other day and it help a lot. I was able to take the dog for a walk. Good luck!
Also a heat pad would help some too.
Heat pad works well. I have an electrical one that I lay on to really get the heat on my back. If you don't have a prenatal body pillow definitely sleep with a pillow between your legs. This will relieve a lot of pressure off of your back. Look up prenatal stretches they too , they help.