Had a root canal two years ago, now gum above tooth and under nostril is tender, had veneers put on. ?

Sound like: The root canal may need to be retreated or that you have another tooth acting up. I would see an endodontist or my dentist for an evaluation.

Re-infection: Sounds like the tooth has reinfected. Best option is to get back to the dentist/root canal specialist who did the root canal and give you options. Re-treatment or a minor surgery in the gum tissue above the tooth may be indicated. I would not rule out a potential fracture of the root, which will present you with other decisions to make. If in doubt, get more than one opinion.