Derek

I agree get it checked out!

In the meantime, two thoughts

I've had occasional subluxation of a rib head that sounds a bit like what you're going through. You can access the area for your self with a tennis ball. Lay on top of it and roll the ball into position on your back so you can put pressure on that spot which can help. I prefer to use my TheraCane to access those areas. Love my TheraCane!

Finally, while you are diagnosing/healing/fixing the area in question, consider changing to a lighter bokken like a yagyu ryu bokken.

If that doesn't work, I've got a four letter word for you ……………… Rest.