how can you tell if your cat is sick?

If you changed her food… that’s likely why she vomited.

Switch your Pets food slowly over a period of 10 to 14 days, if you can. Mixing 25% new to 75% old. Then 50/50… then 75% new to 25% old. And finally switch over to 100% new. Take it slow as not to upset their digestive system.

my cats throw up all the time because of hair balls.
give your cat some hair ball remedy if you are really upset about it.
It is really no big deal it happens to cats all the time.
Only unless he throws up a couple times a day then take him to the vet
other wise cats throw up because when they clean themselves sometimes hair gets clogged up so they puke it up.

Probably hairballs or as Kat said, a new food for you’re cats.