What is wrong with my dog?

My dog has been shaking a lot and he walks very slowly and lower to the ground than usual. If we make any sudden movements like slamming the door or standing up very fast from the ground, he yelps very loud and starts shaking even more. He usually easily jumps up to the couch, but now, he cannot even comfortably jump up or lay down on the ground. We don't know what is wrong with him. He is about 7-8 years old... Do you think he might just be acting like this because of his age?