Why my puppy has diarrhea?

It could just be a change in food that's causing it, and should go away within a few days. You can withold food for a little while to let her system settle, (no more than 12 hours since she's so young), then try feeding the chicken and rice and gradually switching to dog food by putting some rice in the food. BUT if you notice her being sluggish (unusual for a puppy), vomitting, blood in the stool, or if it goes on for more than a few days, I would consult a vet. It's easy for a dog to get dehydrated.