I suffer from intense hip and lower back pain, due in part to 25 years of carrying. Chances are you are irritating the sciatic nerve and or the piriformis muscle, a small muscle that runs diagonally across the butt. A couple things have worked for me: I no longer carry IWB and I tend to use belts that are pliable enough to conform to the waist, instead of concentrating the weight on the belt edge. For example, for range work I use a Wilderness belt, but the narrower, unreinforced model that isnt as rigid but still supports the gun. There are also stretches you can do, but I couldnt begin to describe them here. I still see a physical therapist on a regular basis. Google "piriformis syndrome" for more info.
FWIW my carry gun is a full-size 1911 on pain-free days, a Glock 19 on bad days.