yes they get fatter in the stomach but without a male they wont lay eggs or in the absence of a male two females can pair up and lay eggs.. have you got any other angels in the tank?
Well yes a big belly is a sign of pregnancy, but there is a chance you just have a fat fish on your hands
Angelfish need special conditions for spawning I believe so if you haven't tried to make it spawn with another angelfish you might just have a fat fish.