Neon Tetra fish pregnant??

I got two neon tetras earlier today and they are by themselves for now in a 10-15 gallon tank. I just noticed like 5 min. ago that one of them has a larger stomach than the other. Its pretty noticeable, but i'm not sure if it is supposed to be there or not. i don't know the of the fish or anything. I'm afraid it is a disease , but i don;t know how to tell.. could it be pregnant, only this afternoon it was in a tank with a bunch of other neon tetras so i guess its possible, but i guess its also possible its a disease... any help is appreciated!