tail bone
Yeah, this happens to a lot of us...It came up at a few support group meetings. I think the consensus seems to be that the cause has to do with fat distrtibution and loose skin. You know how when we lay down flat on our backs the extra skin sort of spreads out to our sides? Kind of the same thing with our rears when we sit down, so the padding is spread thin - a "normal" person has their padding held in protecting their tailbone - follow? Kind of like if you lay on an air mattress that doesn't have enough air in it - you hit the floor - or sit in a bean bag chair that isn't "fluffed" right - get the picture?
Time will help resolve it, increasing the size of your glutes through exercise could help by increasing the padding, but most people say they either find different ways to shift and sit or invest in a nice cushion!
One last option, is my plastic surgeon said that if I let him do a circumfrential body lift he would tuck some of the back skin underneath into the top of my butt - no way for me - too expensive and sounds gross, but I thought I would mention it!