Maybe look into a maternity band... I just bought one today for the same reason and once I put it on it helped tremendously! I haven't got to use it more than a few hrs but it really holded up.my belly and strapped around me providing alot of support for my back. I got it at motherhood maternity for $29. They also have them on amazon as well.
I see a chiropractor. She works wonders. I have a back injury from a few years ago and being pregnant just aggravates it. But the chiropractor helps my pain for several weeks at a time.
The best sretch I found was to go on all fours and arch your back up like a cat, and then reverse and arch the other way.
Heating pads are my best friend for back pain and a pulsing shower head
I have the same problem. I finally road my doctor I could not work anymore. I sat at a desk for 8 hours a day and was so miserable by the time I got off. I lay a lot I can only sit for short periods of time.