I would guess there are several causes for most chronic conditions. No fillings and no root canals
doesn't rule out dental infection. I only found mine using EAV. I had not pain and my teeth looked great. My traditional dds would clap when she saw my mouth every six months.
Based off my experience I tell people with m to see a dds who actually finds infections and uses a pano-xray, EAV and Cavitat.
All of that being said I would guess there are several reasons for m. If you get checked please post