Concerned about kitten stool

I've been taking care of my neighbors kitten that was abandoned by its mother a few weeks ago. She's about 13 weeks old. Things were going very well. She was very active and happy. She was having solid stool until today. Today I noticed that her poop is watery and yellowish. I haven't changed her food so I don't think its that. Upon closer examination I noticed that it looked like there were undigested pieces of food in it. I realize that she needs a vet and I plan to take her to see him as soon as I can, but I am wondering if anyone has any ideas of what could be wrong.

It could be any number of things. Is she inside or outside? It could be parasites or worms if she's not been treated for those yet.

Stephanie