Fat fish?

[Zipzap]

Zip my Zebra Danio was earlier thought to be pregnant. Now, she is extremely fat and has been for the last three or four months. Instead of a round stomach area, her belly looks hill-like. As if you were looking down on hills from the air. I am thinking maybe she is getting old? She is extremely feisty and chases the other fish in the aquarium around for no reason. Even my powder blue Gourami is afraid of her.
Any suggestions on what this might be, and what I can do?

Thanks!