My shih-tzu acts weird?

[Ashley]

I have a one and a half year old female shih tzu. The past couple of weeks she has been acting weird and not as affectionate as she usually is. She hides under everything, my bed, chairs, in her pet carrier. She barley ever comes out unless I have food. The vet said it might be because she is in heat, but I haven't seen any blood or anything from her. I have another shih-tzu and she acts fine. If anyone knows what might be going on please let me know. I am worried.