I used to have a Shih Tzu named Jasmine. I loved her dearly. She became listless and would not eat unless I fed her out of my hand. I took her to the vet and she found nothing physically wrong with her. She asked if anything had changed in her environment that could possibly be causing her to be depressed. Something had changed. My daughter's cat was staying with us because he was not allowed in her dorm room. Jasmine was depressed! Shih Tzu's are very emotional and they get upset easily. My first suggestion is that you should take her to the vet immediately. Don't wait because Shih Tzu's are very delicate. In the mean time, give her lots of extra attention.