My dog (a shih tzu) gave birth to one puppy two weeks ago, she's been having diarrhea, what to do?

From my understanding, it is natural for the mother to keep the den clean by eating and licking the puppy's wastes. Would this be the reason why she's been having diarrhea? Also, I've changed her diet from adult BLUES to puppy Royal Canin for more nutrion after birth. She's had Royal Canin when she was a puppy and there were no signs of diarrhea. And also, she didn't eat her placenta after birth. Any answers would be helpful and appreciated. Please!