Can Bad Posture Cause Rib Pain?

i have this pain on the left side of my ribs. it only hurts when i sit up for a long period of time. sometimes it takes an hour or two sometimes more time. and its only on my left side. is it possible for this to be because of bad posture. i always slouch and don't sit straight up. so i was thinking it was a possibility. i did go to the doctor and she couldn't find anything wrong. i even got x-rays. so nothing is physically damaged, any ideas or suggestions?