I think you're absolutely right about me having a hip issue. Two years ago I torn my hip flexor on the other leg. However, my back is the complete opposite of being flat. In fact the lower part is very curved and it kind of makes my buttocks stick out more. When I lean against a wall, if I flatten my lower back on it, my upper body slouches alot. I think it's supposed to be like that, but not to the extreme that my back does.