My dog is acting odd?

A few days ago, I came home from work to find my 3 year old Dachshund/Shelby Sheppard mix stuck under the bed, thrashing around. I'm pretty sure it was due to thunderstorms. After I let him out from under the bed, he was acting very strange. He was panting very heavily and drinking a lot. He seemed very confused and almost scared. The next day, I got a text message at work from my fiance saying that he fell down the steps twice. We took him to the vet but he couldn't find anything wrong. For the rest of the day, and all of today, the fearful/anxious behavior did not stop. He is twitching a lot. He is also tilting his head to one side and he is walking very wobbly. He is a rescue dog so I don't have a lot of information about his medical history except that he has had problems with ear infections (I clean his ears out regularly) I do not know what is wrong with him and this behavior doesn't seem to be getting any better.