It doesn't sound at all baby related, and if the back isn't against your belly button (like its to the left or right) its normal that its not so stretched out it couldn't fold. With regards to the pain, call your midwife if you're worried, but it sounds like you have just pulled a muscle, especially if you are completely pain free between sneezles.
YOu body is designed to carry your bub so you doubling over is completely safe too.
I hope that helps, but truly if you need piece of mind call someone, they won't mind at all!