Well, I'm not going to be an asshole about this, but I am going to be honest while I critique.
I noticed within the pink you forgot to draw her a pelvis, THIS is why you're having a problem with her ass AND her legs.
*Before I continue, I'm going to be using "her left" and "her right". When I say this, imagine it's your left or right, ok?
Her left ass cheek is sticking out WAY too much compared to where her torso is (also why it's crucial to draw a pelvis). Shorten it by a fingerwidth (aka your finger, compared to the drawing) or less on the side (all while keeping it rounded) to make it seem more realistic. Because there's no pelvis, it's also hard to tell where her right thigh would begin, and where her stomach and crotch/pussy wold end. You should bunch the right half of her shirt/dress around her hip, to also give the illusion of foreshortening. And lastly, her right knee looks like it and the rest of her distal (further down her knee) right leg was twisted inward, and makes it look really inconsistent with her right thigh.
Sorry if I'm brutally honest; Otherwise it DOES look good, but you were right to ask if there was something off from her butt and legs. I hope my advice can ehlp you, somehow (otherwise.... I made myself look like an art-nazi at worst)