Go to the emergency room immediately. If you are experiencing severe pain that could indicate the beginning of an infection. The faster you get it treated, the less likely it will result in something drastic, like amputation or death. You should also file a police report, just to have an official written record of the incident. I would also report the matter to animal control. Dogs that aggressive should never be left out in a yard where they can get to someone and hurt them. I have a dog, and I love him dearly, but I would never leave him unleashed outside because I know he's a bit unpredictable. In addition, dogs like the one that attacked you put other dogs and people at serious risk. PLEASE report this ASAP.