What should I do with my sick cat

I adopted a cat last Sunday who was spayed on Friday. She's an adorable one year old calico and we were given vitimans to give to her daily, which we did. They told us at the humane society that she was used to IAMS brand of cat food, but she wouldn't eat it. Now, all she eats is the Dad's brand and she has HORRIBLE diarrhea. She goes on EVERYTHING. She will eat every five minutes, literally. Then within the following hour or so she GOES! I would feel like an if I gave this cat back to the humane society. She also had an upper respiratory infection which we are treating with antibiotics. Could those meds be making her sick? She eats everything in her path and gets sick from it. WHAT SHOULD I DO? I like the cat, she's gorgeous and kind of gets along with the others but we can't watch a poopy cat 24/7!

Your cat has just gone some HUGE uphevals in her life. She just went through a pretty intense surgery (my little girl was off for days afterwards), was rehomed (while this is a VERY good thing it's still a big change especially with other cats), is on antibiotics because she has been unwell & has changed diet. All of these things would likely affect her tummy. If you haven't already I'd keep her confined in the laundry or bathroom with water, food, litter box, bedding & maybe even a couple of simple toys. This will help her to settle into one room of the house so she feels safe & will help keep her quiet while she heals - you should go in & visit with her a while, making the visits longer each time - it will give everyone time to adjust to the new member of the household.