I have had exactly what you're talking about on my left middle finger from constant ADS'ing around corners and when i press down to sprint, which i believe causes it. After i got a Scuf controller it went away. It is most likely due to the Scuf controllers grip design which is designed for actual gaming. The stock Xbox controllers are simply trash and are really only designed for light or casual gaming.
So my advice is look into getting a Scuf controller. I promise you will not regret it, hopefully this helps.