I also have experienced the same back pain as you have described. It is like by my bra line and there is absolutly no way to sit down comfortably. One day it hurt so bad that I started feeling tightening braxton hicks. I decided to go in just in case because I am at risk for preterm labor. They ran the normal tests and baby was fine. The nurse said sometimes pregnant women can have a disk pop out during pregnancy in your back. there is really nothing we can do but feel like crap. So my sympathy goes out to you too.