I have been a trapshooting coach for several years now and like you have shot a shotgun all my life.. I have never heard of anything like this.. not even from the smallest kid who cant even hold his gun up right.. when the gun fires the recoil of course goes backwards along with your shoulder and your hand that your gripping with.. unless you have developed some type of flinch from those 3 inch shells.. (which I would not doubt..) I cant imagine what it could be.. I have seen bruised and bleeding cheeks and shoulders from ill fitting guns but never a middle finger.. once it gets better.. start using the middle finger for your trigger finger and see how it works.. its a little awkard but very doable..

Heres the problem your pulling the trigger with your index finger at or past the first joint, just use the tip of your finer to pull the trigger and it will go away, took me a year to figure it out and 2 months later it still hurt

Its a Rem 870 supermag. Always tip/pad of the finger, no joint or furtherback.

I'd soak it in something warm and moist!!