That's the only thing I can figure. I would recommend using a different grip. This happened to my buddy because he had a revolver-style grip. Use a modern 50/50 grip to solve this. Check this video out, it has improved my shooting greatly and no slide biteMaybe slide bite on the thumb?
On some guns, the edge of the trigger guard where it meets the frame is too sharp. Take a small file and round it off and it will stop rubbing your social finger.As it was explained to me. Your middle finger goes under the trigger guard on the high point of the grip while firing. After shooting for a while you get a blister right in front of your second knuckle of the middle finger. I asked my instructor how to prevent it and what I was doing wrong. He said " nothing, it means you have been holding it right" and he called it "glock knuckle". I thought it was common, I guess only he and I have called it that.