What's wrong with my dog's swollen back leg?

My roommate noticed that when she touched his back right leg that he would yelp and try to bite her this afternoon. When she told me I looked at it and I noticed that it's swollen. He can walk on it fine with no problems, it just when you try to touch it that he screams. He's a 10 lb Chihuahua and his front feet are deformed, but there's nothing wrong with his back feet. He jumps off of couches and stuff and sometimes falls off, but it's never bothered him before. Should I be worried? I'm taking him to the vet tomorrow, but I'm wondering if I should take him to an ER vet...

Thanks so much for your help