extreme back pain at 15 weeks.

Courtney
I am 15 weeks today and can hardly walk due to my backpain. The lower middle part of my back all the way down to the bottom of my butt radiates with pain and it's just a giant, tight knot. 
Do any of you new mommies or mommies to be know of some remedies to ease the pain in my back? I've tried stretching, walking, and laying flat on the floor. Laying flat seems to be the only thing that has given me any relief. I work 8-10 hrs a day sitting down and can't lay down. I have avoid taking any medicine since I found out I was pregnant, but I am miserable. 
Help?!