I have symptoms of sinusitis with mild facial swelling. A year ago, I had a root canal on the same side on the upper molar number two with a crown placed. That tooth is tender now. What could be the cause of swelling, tooth or sinus? I am not sure whom to

Dentist First. From you history, i would start with the root canal tooth, then work from there. It is possible, given your timeline, that the root canal possibly failed, and the infection recurred. If so, treatment is necessary and may allow you to still save the tooth. It is possible it's a sinus infection, but my first guess would be the rct tooth. You want a "pa x-ray", digital if possible, or ct scan.
Root canal failure. Go to your dentist or endodontist to have the tooth evaluated. Root canals can occasionally fail. Depending on the situation the root canal may have to be retreated by an endodontist, or surgery called an apicoectomy is sometimes done depending on the position of roots and their relation to the maxillary sinus.
Start with dentist. Tooth abcess should be ruled out. Sometimes if a post is put in , it can be placed in such a way as it is extending in to sinus. This can cause an infection of both the tooth and sinus. Xrays and sometimes a cat scan is needed. An ENT dr would then be needed.
Dental consult 1st. The facial swelling would tend to indicate an infection is present. First see your dentist to diagnose the cause of the problem(s) and suggest treatment options. The roots of that tooth usually approximate the sinus and you may be getting referred pain, inflammation or infection of the sinus or an unrelated issue. Your dentist might recommend you also see a physician.
See a dentist. Ther is not usually swelling that is visible with sinusitis. You should see a dentist to see if the tooth has re-abscessed.