What could be the cause of the swelling that I have in the gum around the area where I had a root canal?
Infection. Swelling usually means infection, but not always. Could be from untreated accessory canal, split tooth, gum disease, infected adjacent tooth, or other reasons. Go back to your treating dentist to have situation reviewed, diagnosed, and treated.
Gum swelling. If you have swelling in your gums around a tooth that has had root canal, it come be a root canal that is still infected. It could also be a periodontal infection or a gum infection. In either case a visit to your dentist to evaluate the problem would be a good plan. Good luck with it!