I've noticed, for the past three or four weeks, that the tip of my left index finger has gone numb. During this time of not-entirely-consistent practicing, I've been pressing down on strings fairly hard. I'm still very new to this, don't have the hand strength just yet. I've also, until very recently, been severely bending my left wrist in order to maintain my thumb on the back of the neck. I'm stopping that foolishness now. I guess I'm wondering... is this something that could have been brought on by overenthusiastic playing? I'm not noticing anything like this in my middle, ring, or pinky fingers... and I press down with those, too. Is it possible there's something more serious going on, or is this the initial stage of building up callouses? Thanks!