Please help! What is this knot I have beside my root canaled tooth?

Swelling. Any swelling or bump by a tooth that has had a root canal is suspicious of an infection. See your dentist and get this checked.
Parulis fistula. Is a sign that there is an infection and your body is draining it out. Unlike some other types of infection, a dental abscess will not get better on its own. See your dentist for definitive diagnosis and treatment. Good luck.
Abscess? Can't tell over the internet. Even if you don't have any pain, you should have it looked at by your dentist.