Have you reached out to your doctor about it? Could it be gas traveling that you might be getting from a specific food your eating? The only time I've had chest, back or waist pain was from gas after surgery. Does it hurt when you exercise? I would definitely call the doctor and let them check things out. It would have been more severe a long time ago rather than after the weight loss and the length of time. Congrats on your 100 lbs.!
My sides hurt when I'm in the car. I think it is from the pressure on my body, I don't have the cushion of fat protecting me. Even sitting on a wooden chair is uncomfortable.