Irrelevant

So I know this is irrelevant with anything on here but I need help. My chihuahua is (in my eyes) really skinny. I can literally see her spin and tail bone. I definitely feed her but its like she's forcing her self to eat. I gave her a little of my food one time and she ate it like she never ate before. I changed her food but I'm on a budget and the good foods are usually expensive. How can I make her gain healthy weight?