HELP! My Dog's Tail Has A Bump On It and He Can't Even Wag It!?

Last night my dog was fine. I woke up this morning and called him to me and he came but when he wagged his tail, he shrieked and went back to my moms room. We both look at his tail and she found a bump. When she touched it, he shrieked again. We are both very worried. Please Help!!