If you feel she is not spayed then I would plan on getting her fixed soon. That will help prevent her from getting pregnant in the future and also it will eliminate her risk of developing a condition called pyometra (spontaneous infected uterus). Her belly fat is probably normal, so unless she is considered to be overweight then continue with her normal diet (Hill's Science Diet, Iams, Evo (Innova), adult cat food). Here is a website link to help you determine if she is ideal weight or not
https://www.wsava.org/sites/default/files/Body%20condition%20score%20chart%20cats.pdf OR you can have her examined by your local veterinarian.
As long as she is eating, drinking, urinating and defecating normal, then she is likely fine. Just check with your local vet to make sure she is fully vaccinated to protect her from preventable diseases. Best wishes with your newly adopted pet.