I think I figured out how I got it. I switched my main back exercise to plate-loaded Hoist pull down and it has pretty thick handles. I have a strong grip too, so I hardly ever use straps and I think gripping that thicker handle while pulling heavy weight did it. I stopped doing that machine and the pain was gone when I did back a few days ago. I'm gonna have to use straps for that machine in the future.
So, if you're not using lifting straps, then that might help (especially if you're doing back twice a week).