I found out from my dentist that even swallowing can cause many, many pounds worth equivalent
in pressure
on the tiny
jaw joint. And the afternoon sounds like it may be as time goes
on, and you have exercised the joint and muscles more, the issue has been exacerbated (aggravated) by usage. Also, you may want to focus on relaxing the muscles in your face, and go slack-jawed and see if that helps with the muscle stress aspect of things as well as taking pressure off the joint.