What's wrong with the stray cat I found?

[Unknown]

I picked up a stray a few nights ago. She's maybe 6 months old. She's very lethargic and had diarrhea which is subsiding. Her stomach is still swollen though. Also, she is refusing to use a litter box. Please help!!
Also, the cat is eating and drinking plenty. And she doesn't have a fever.
I'm taking her to the vet this week.