Maddie has done this before and ended up having a partial blockage. Her vomit did not only look like poop, it smelled like poop. YUCK! She ended up with an infection but was treated everything turned out to be ok.
Cooper has done this and it was nothing. But, she only did it once where Maddie did it a few times and off to the vet we went. I think Cooper had eaten poop, probably cat poo from outside. Lordy, it was smelly.
If it has only happened once and everything else is normal (bm's, eating, drinking) she is probably fine. They can eat something so fast, you may have just missed it.
If she does this again, I would take a trip to the vet just to make sure everything is okay in her digestive tract.