Lisa,
 
I bet it's something she ate.  If so, now that you are there to resume
care, her poop should return to normal by tomorrow.  I have found that
feeding pureed chicken gives my guys runny yellow poop once or twice.
Yellow tartar control cat treats turn the stool yellow.  Red cat treats
turn them red (THAT was a scare, first time).  Of course, if loose stool
continues, or if they aren't eating and drinking normally, then yes, see
a vet.
 
cb
[Posted in FML issue 3424]