Sounds like a broken rib, which if it's separated (not just cracked) could seriously injure you. 'Popped in and out of place' is an odd phrase for a rib as it's not a joint. Get to a doctor and get imaged they can't really do anything for the butt so I would worry much about it.
Ribs do have joints, just not fully-mobile joints. They allow flexion during respiration (or else we wouldn't breathe very well). Does it hurt in your back near your spine like a knot in a muscle or on your stomach side? I've dislocated ribs in weird ways before and it sucks, but with the right stretching/massage it will come-to. If you inhale fully and really puff out your chest where does it hurt? If it's not near the spine then you prob need an x-ray because it could be broken. It all depends on where it hurts though.