17yrs old - behind my 5th tooth(counting from right side front tooth) on the gum, for 1wk or so i have what feels like a deep cut or canker sore?

Options: based on your description that is an upper right 2nd bicuspid or tooth #4, If behind means on the palatal side of the tooth, it could be an ulcer or abscess. If behind means close to the tooth behind it, it could be a gum or bone problem. Suggest you see a dentist for a definitive diagnosis and treatment

Canker sore: canker sores usually cause minor nuisance, but can be associated with other health problems, can be recurrent The exact cause is not known, can be related to emotional stress, lack of sleep, mechanical trauma, certain food and chocolate, certain toothpastes and viral infection You can ask pharmacist for advise, Seek medical attention if persist more that 3 weeks or if disabling