If it's uncomfortable for me to sit straight up, is there something wrong with my back?

Whenever I sit straight up, it is really uncomfortbale to me and i get exausted, but when I slouch, I dont feel an kind of discomfort. I got checked for skulleoisis (or however you spell it) at school twice, and both times they said I didn't have it. But then again, they were PE teachers checking my back. Is there anything wrong with my spine or something and should I get it checked out?