OK i didn't have much time earlier when I posted and I see that it wasn't hugely helpful.

Every time people do things like walk and sit down they often do it in a bad way that the body isn't designed for. Do you lean when you are on the computer? Are you leaning now? SIT UP STRAIGHT. That said there's a correct way to sit up straight, and it shouldn't feel to uncomfortable though it may do at first ecause you aren't used to using those muscles.

I was lucky enough as a musician to have Alexander lessons to stop myself doing harm to my back while practicing. I know learning from a book will be different, and I don't know how succesful it would be, but I highly recommend you give it a shot.