Well, I don't know what it is, but I know that I get really bad back pain when sitting too long, but my job is in a call center so I sit for about 6.5 hours a day.

Can you go for walks? or try stretching? I know when I stretch at night before bed, I tend to sleep better. Also, what about a nice bath? If you are stressed out maybe you need to get a really good book, put something that smells good to you in the water and spend about an hour in there with no distractions. It helps you relax. I have tried this a few times when I have been really really stressed and it works...at least for me.

I hope you feel better soon.