Could be. Or from a virus from a friend, or from eating a bad hamburger, or from a dirty door knob at school, or from a pet turtle, or from the dog licking her face. Could be just about anything. Have her push fluids. Gatorade is good. If any severe pain, bloody diarrhea, fever, or if she can't keep fluids down, need to get her to doctor ASAP.
And make sure she is doing good handwashing, which would prevent most all GI bugs.
Salmonella would cause diarrhea, BUT it takes about 3 days from exposure to the bacteria to develop symptoms. Chickens don't just suddenly appear with salmonella, so unless you got the chickens in the past week, I'd bet its one of those unavoidable childhood illnesses.