Five years ago I had something similar. Dentist x-rayed and found infection. Neither he nor endodontist who'd done the root canal for that tooth could see the cause and guessed cracked tooth. Then endodontist referred me to a periodontist who did a gum flap so he could see under the gum to the roots of the tooth. He discovered that one of my roots (a molar with three roots) was cracked. He then performed a root amputation where he removed the cracked root, filled the gap left with a collagen insert, and sewed the gum back together. It occasionally aches, but that could be caused by my frequent sinus infections.