mum2be You probably have been but: Have you been doing pelvic floor exercises? Am and Pm if you can. I can tell when I've been slacking off as my back hurts a lot more (like today, fell asleep doing mine lastnight).
Also, try the pilates cat-stretch (on all fours, pull up pelvic floor muscles and in tummy muscles [as far as you can go] push your breath out gently as you arch-up your back and hold for 10 seconds, breathe in as you 'relax' back, and repeat 8-10 times. You can build up the no. of seconds and the repetitions). I'm finding this much easier than the curl up/down that you can do.
Also, lie on your bed, on your back, bring legs up and feet fairly close to bum. Engage pelvic floor and bring in tummy muscles again. Breathing out, drop legs togeher (like they're glued) to one side slowly and gently allowing your sides to stretch out (roatate with your feet touching the floor), hold for a count of 4-8 (breathe IN for this bit), bring legs back up gently (breathe out), rest at the top (breathe IN), and do the other side. Repeat 4-6 times for each side. If you want to make it more difficult you can raise your feet, or even stretch up your legs
. (You're essentially rotating your spinal column so you're on your hip during the stretch but on your back when legs are back up). If you can't get your legs all the way to the matress, pop pillows/cushions under where your thighs or knees will be to stop you over-stretching your ligaments.
I love both of those to stretch out my back (muscular injury). Always have and these two are still proving useful. NOW, be careful with this one, as it technically counts as being on your back...so any signs of feintness when you 'get up' and dont' do it again...which we're told to avoid, but I can't see this as being too bad, as you're 'moving' the lower back and if you only do it 4 times on each side, you're not on your back for very long!? I'm certainyl waking up having been on my back for longer and feeling fone. Either way, my back's loving it for now. IF in doubt, ask MW who shoudl be able to give you some stretches too.
Re: lack of fluid. If worried, talk to MW, but this seems unlikely, as you've not been leaking fluids and all was OK at 25wks apt (you'd assume they'd pick that one up). LO's prpbably adding to the pain with being low down, but, well, try a hot water bottle throughout the night. And, if it's still happening for a while after you get up, try cold compresses too (both will ease the ache). Try also sleeping on left, but padding out your right side/back so even if you lean back, you're still not really that far from left (IYSWIM). I've found my furrbabies act as good back padding 
Sorry for the length, but, well, hopefully it'll help...maybe?!