On some new Glocks, there may be a tiny bit of sharp plastic flashing (mold overflow) on the trigger safety lever or face of the trigger itself. Under recoil, it tends to chew on the shooter's trigger finger, making it tender/sore. Does this sound like the problem you are experiencing, or is the trigger actually smacking your finger hard enough to bother you, even though it is smooth (no sharp spots)?
If it's the plastic flashing, I usually use a small piece of medium-fine or fine sandpaper to smooth any sharp edges on the trigger face or trigger safety lever. I've never experienced the trigger slapping my finger on a Glock, so I'm not sure what might be causing that, if that's what you're feeling.