Every 2 months or so, my molar crown begins to hurt badly. It last about a week and then fades. X-ray didn’t show an issue

It’s worse when biting down. Sometimes it’s so bad I can’t chew on that side and I also get headaches. It feels swollen and painful. There no visible issue, just the pain. Sometimes I wish I could rip it off.

I had a root canal. What could be the issue?