If it's any consolation I got to around 5months and my back pain was agonising, and I was so worried about how I would continue to work as I assumed it would get worse as the pregancy went on but it seemed to clear up and it was fine (or as fine as a pregnant woman's back can be!) from then on. I think it may have just been baby's position and as she got bigger she moved and so took the pressure off.
I would check about them warning you re taking sick leave, I know workplaces differ but our place weren't allowed to pursue sickness absences any differently to when you weren't pregnant, except in the last four weeks of pregnancy whereby if you go sick with a pregnancy related illness they can begin your mat leave from that date (in this case back ache would probably start your leave early, but a cold wouldn't). I worked until I was 39weeks but went sick at 37weeks for four days as I had an awful cold and chesty cough, I returned to work and then left just before my due date.