What is wrong with my new dog?

My fiance and I recently adopted a one and a half year old beagle. He's very mellow and sweet. He walks around with his head down and he seems pretty sad/nervous. I know that's pretty normal when someone adopts a dog, because they aren't familiar with their new home and people yet. We adopted him from a family that was going to send him to the pound. They seemed nice and honest. But today (we've had him for five days) he started shaking and his heart is beating so quickly, it can be seen in his neck. When my fiance was petting him, he yelped when his hip was gently touched. He wines a lot. I don't know what's wrong with him. Any help would be appreciated. Is this normal for a new dog?