what is up with the BOOTY?

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So I lost 20lbs in less than 2 months and almost ALMOST down to the same weight I was in NYC 2 years ago (NYC was home, moved to Ohio, traded 3 miles of walking a day for a Hyundai). I've always been extremely, naturally thin... however... I gained weight from not exercising/I gained weight pretty much everywhere! It's certainly shedding off my body pretty quick BUT my thighs and butt refuse to shrink!!! I've noticed the cellulite is pretty much gone, so I guess I am toning in that region and all (no complaints). All I am doing is tredmil with an incline and the elliptical... why isn't my body responding in these areas? Am I building muscle under the fat?

I've never had a booty before! Sorta cool to have but I could go without the thighs. Anyone have any advice on how to "fix" this?