I've never had a cavity and always take great care of my teeth. Now I have lupus, got gum swelling and lost a back tooth. Could lupus be the cause?

Possibly, but rare. It sounds more like periodontal disease. You state that you take great care of your teeth and never had a cavity. That is great, but you did not state whether or not you see a dentist regularly. If you do not then the swelling gums and lost tooth is most likely due to periodontal disease and not lupus. If you see a dentist regularly then go to see him/her and get this checked out.
Possible association. Lupus could have been associated with the tooth loss. Was there bone loss and a loose tooth? Do you regularly see your dentist? Did the tooth have a fracture? Sometimes it's hard to pin the cause on any one factor. Talk to your dentist about this lupus situation.
Auto-immune. As you know lupus is an auto-immune disease. Determining what caused the swelling and tooth loss requires you to see a periodontist (gum specialist) to see if this was also an auto-immune problem, or another type of gum problem. Please ask your dentist for a referral.
Lupus Diagnosed? Have you already been diagnosed with lupus? If so, yes it could be a factor. If not, it is not very high on the list of possibilities by itself.

Related Questions

Severe swelling around front and back tooth. Gum swelling. No pain and no sensitivity to hot/cold. How can I reduce swelling till I go to the docs?

Swelling. Swelling usually indicates an infection, either from the tooth or the gums. Warm salt water rinses may help (dissolve one tsp of salt in a small glass of warm water), but is not a solution even if the swelling subsides. Any swelling needs to be seen by your dentist. Read more...
Gum swelling. Just go to see your dentist. Tyou need a professional care, in the meantime floss and brush as mucha as you can. Read more...
Gum disease? You might be suffering from gum disease. It would be wise to see your dentist. In the meantime try to keep your mouth as clean as possible. Also try rinsing with salt water until you see the dentist. Read more...
Gingival abscess. A gum abscess usually requires treatment; otherwise it can worsen and result in the destruction of the surrounding tissue. See your dentist for evaluation and treatment ASAP. Read more...