I have a large canker sore that is very painful and I also noticed that a tooth nearby has all of a sudden developed a black line at it's base. Ideas?

Two problems. 1. Canker sores are the most common oral lesions. The cause is unknown. These lesions are bumpy and painful. Topical medications OTC applied directly, mouthwashes, and oral medications can relieve pain or inflammation. 2. Black line stains are calculus, decay or a simple stain. Re.: visit your dentist to solve this small problem.
See Dentist. The canker sore will resolve on it's own. See your DDS to assess the tooth.