Why does my American Bulldogs backside look realy skinny when standing but when she lays down she looks normal?

I adopted my 4 yr old American BullDog from a lady in my city. The problem is when she stands up her backside looks really skinny but when she sits and lays down she looks normal. I feed her 3 times a day, Gravy-Train.(Thats what previous owners feed her) She eats her food just fine and she uses the bathrooms just fine. We have also given her worm pills to make sure she doesn't have worms. I was just wondering if this sounds like something serious or maybe just what her body type is. Also she is fixed and UTD on her shots.