I second that it sounds like sciatica (sp). What you describe is EXACTLY what I felt with sciatica when I was preg with P.

It sucks...but I don't know how to fix it. I was working when I had it and sitting in my office chair was agony! I just put up with it, slept with lots of pillows supporting my back, knees and belly and stretched frequently. Didn't stay in one positon during the day for too long.

However, it disapeared as soon as I gave birth...so...hopefully...you only have 5 months of it to go... :-(