Hi bluebird, sorry you are on such a tight budget. Sounds like you need a helping hand. These are ways to help put more in the budget for essentials like food and cat food.

These are only ideas but maybe some of them will work,

Food stamps
Food pantry
Free cat food if you are on food stamps Can't Afford Pet Food? Pet Food Stamps Is Here To Help | Care2 Causes

Part time job - there are Clubs around US that help people get part time jobs that enhance SSI and other programs