pain only when i don't work

I'm a 28 year old female, and have been experiencing back pain only on my days off, and only while sleeping.  I work in manufacturing and packing, and I'm on my feet at work for 10 hours a day.  I work nights, so I'm getting home around 2am, but I have no back pain when I wake up, I sleep for about 8-9 hours on the nights that I work.  But on my days off, without fail, I wake up with severe back pain, making it hard to walk, yet alone breathe.  I'm 5'9", and roughly 190lbs...I bought a new serta pillow top mattress last year, but this pain seems to have started roughly about the same time I started working nights (I used to work day shift, and I can't remember having this pain during my days off when I worked the day shift). This is all very confusing to me.  Why would changing your shifts cause back pain? My back doesn't hurt while I'm at work, or even after work...Is there some way that my muscles could be over relaxing on my days off? I have 3 small kids at home, so it's not like i'm sitting on my duff all day long on my days off...just looking for some insight...hopefully someone has some ideas.

[Swivel]

I have chronic pain in general and I keep myself moving as much as possible. It seems like when I rest or sleep I get up feeling worse. In my case my pain in Chronic Fatigue/ Fibromyalgia most likely cause by the Epstein Barr Virus that I contracted. In your case it could be totally different. I still wanted to share a recent discovery with you. Before I go to bed or plan to sit for a bit I stretch a little just to loosen up a little because for some reason my pain tends to build up while relaxing, I figure this gets rid of some of the pain I might feel later. You could also try exercising your back a little just to make sure its strong enough for the type or work your doing. I dont think that your hours would have much to do with it unless its extremely stressful for you and your family then I guess it could be the reason for everything. Stress is a monster. Hope I could help a little.