Why would my tooth that i had a root canal on suddenly start hurting?

Inflammation: A tooth can sometimes be sore for the first 48 hours after a root canal is completed. Cleaning out the nerve of the tooth can inflame the area in between the end of the tooth root and the bone. Ibuprofen and 48 hours should make it feel much better. If the root canal was done a long time ago, it could be hurting because tooth is cracked, grinding on the tooth, or even sinus issues if top tooth.

Many things: Most likely to be an infection or a fracture. In the upper jaw a sinus infection can also be suspected to be the cause. Clinical exam and radiographic evaluation is advised. See your dentist. If the rct was recently done there can be many other reasons.