I think fizzie's suggestions are good ones.
In my mum's case, she often sits as you describe. I have found two main causes: pain such as headache or backache, which might be caused by kidney trouble, or even by poor posture; and just not feeling at ease in her chair. This is sometimes improved by putting a pillow behind her and 'propping her up'. A footstool might also be helpful, but only when someone is with her, as it's so easy to trip over these!
As fizzie says, it could be a whole range of things.
Good luck, I hope you find something that helps
Lindy xx