It's around my medial epicondyle, feels like golfer's elbow; like the kind of pain you get when you squat with your elbows under the bar instead of "up".
Do pull-ups bother it?
No, just chins.
Pain on weighted chins is not uncommon, especially if you go from unweighted to something like 25 pounds with no build up. How did you load these? Do you do weighted pullups, too?