My dog won't stop crying and shaking?

I have a 1 year old shih tzu, and she’s the cutest thing. She was sitting on the stool in our living room and all of a sudden she started crying and whimpering almost. I picked her up and she was shaking. It was out of no where and nothing I knew or saw triggered her to cry like that. She’s been crying all night and she was in my room on my bed crying and just staring at the walls. It was really weird. I just went into the kitchen to get something to eat and she followed me, like she always does, and sat on the stool again. She was crying again and shaking. I’m getting really worried because I don’t know what’s wrong with her. Could she see something I don’t? Im so scared. Please help!