My babe (12mos) is doing the same thing right now -- he's got some slight gross motor delays due to low muscle tone, and has just recently learned to get into the sitting position on his own. I used to lay him down and snuggle him on his side w/2 rolled towels on either side...It was finally all working beautifully!
And.... now he immediately gets into sitting, and starts squawking. He doesn't like being on his back, and isn't crazy about being on his belly as he can't roll from belly to back.
To make it more nervewracking -- his new prefered position is to nuzzle up to his bumper, half on his side, half on his belly, almost face down, face practically buried! Of course we're worried he'll suffocate... not sure if that's laughable or real.
I really hope he starts to love being on his belly OR learns to roll! We were up from 10:30-11:30pm and then 1:30-3:30am w/him screaming b/c he was so tired and couldn't get comfortable.
The fun just keeps coming...
It's a really good thing they are so cute.