[ Dogs ] Open Question : Terrier is very sick?

My rat terrier who is maybe 6 years old is having a hard time walking around, she cannot jump on and off couches without being in pain. She yelps when i touch her sides, plus she does not want to eat anything. What could be wrong with her? She jumps over the fence alot, so she could have torn some muscles maybe? She has been hit by a car twice now, so that could be another possiblity. Any other suggestions?