I had the same problem with my middle finger only, shooting 3 under. I first experienced it when I switched to a tab from a glove. The glove had some fairly stiff cordovan and the tab just thin leather and calf hair.
Arnie Moe, moebow on the shooters Form forum, explained there is a nerve which runs near the surface of the inside of the first joint. Some people are more sensitive to it than others.
I was carrying most of the weight of the string with my middle finger - like 60% middle finger, 30% index finger and only 10% on the ring finger. I started using a little deeper hook on the index finger so the string was in the joint of both the middle and index finger and that cleared it up for me. I did eventually go back to the glove, though.
Unlike you, I get a better, cleaner release and can keep my hand more relaxed with a deeper hook.