This sounds exactly like what I had at 36.5 weeks. It would come and go and was excruciating. Hubby came home a few times from work because it was so bad. I was told it was muscle. Anyway it turned out to be pre labour. It lasted a week and a half and at 38 weeks it ramped up, still all in my back. I wasn't sure it was labour but then I felt waves of the same pain and 3 hrs later DS was born. I can't see how far along you are but if its pre labour hopefully it means your body is doing most of the hard work now. Good luck
Babyluv, I am 36.5 weeks!! LOL