Why do i feel discomfort after root canal therapy if the nerve is gone?

Phantom pain: When nerves are injured there is sometime a condition where there is "memory" and if the nerve is gone, the rest of the nervous system remembers the nerve and there is the sensation of pain. This is a complex and poorly understood conditon. It happens with loss of limb as well. It also could be inflammation in the root canal itself. Discuss with your dentist or endodontist.

Periodontal Tissues: Yes the nerve in the tooth is gone, but the pain comes from the bone and gingival tissues which are still very much alive and have sensation. Biting pain is the most common and that has nothing to do with the nerve in (was in) the tooth.

Possible infection: Root canal therapy is necessitated by the death of the nerve inside the tooth. This is often caused by bacteria in the nerve...An infection. Often, this infection migrates to the opening at the tip of the root and from there into the bone outside of the root. This infection can persist for awhile after the root canal procedure is completed. It is often accompanied by pain and swelling.

Micro trauma: If the tooth area was inflamed before the treatment and the treatment was extensive or complicated, it is possible that the gum or ligament of the tooth can be inflamed and need several days to calm down. Bite adjustment may be necessary, speak to your treating dentist.