This topic comes up from time to time. It can be hard on your body if you don’t have the fit correct. I have always recommended to people that the hips of the seat need to be ” snug” not tight but “snug”, and the upper rib areas need a bit of clearance. If you wear a rib vest and its tight and then you “squeeze” into the seat when you start to breathe heavy your chest cannot expand. This puts a lot of stress on the fibers between the ribs and creates a lot of pain. Try not to hold your breathe through a turn, it will only make it worse. Instead, as you approach the turn take a breathe then breathe it out as you go through the turn. This will give you the clearance as well as tighten your core which will stabilize you in the kart. After awhile it will become second nature and you wont have any issues. Hope you feel better soon.