Waking up with stiff & sore back?

I'm having trouble getting enough sleep lately because after about 5 or 6 hours in bed, my lower back and hips are becoming so stiff and sore I can't get comfortable in bed. Anyone have this problem? Ideas of what's causing it or how to fix it?

I'm 28 years old, stay-at-home mom of 3 kids. My youngest is 8 months old, and sometimes ends up in bed with me nursing for part of the night. For a while I thought the back soreness was a result of sleeping in strange positions with a baby in bed, but last night he was in his crib the whole night. I got up once and nursed him sitting in a rocking chair, then put him back and went back to bed. I was horribly stiff and sore a few hours later and I'm still working it out now. I've never had back trouble before. I usually go to sleep on my side, but sometimes I end up on my back, particularly if I have baby in bed with me. I don't workout/exercise very often just because I'm so busy with the kids, but they do keep me moving and on my feet a fair amount.