Hey there! I saw the pictures and I honestly can't tell if it's fat or loose skin. You sound very slim at 5'9'' and 135 pounds (especially for a male) but I know that sometimes that won't matter. It can be VERY hard for guys to lose their stomach fat much in the same way it's hard for a female to lose their thigh fat. It's a curse of genetics I'm afraid.
It could be skin though :-/ Maybe someone here that has lost a fair amount of weight and has gone through the same thing will be able to tell you