A burn mark or blood on tip of the nail of the thumb which may turn black after a while. This  usually means you have too  little reverse, or a span that is too long.
 It is also a sign you are hitting up on the ball at the release point.
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MOTIV GIRL

Edited on 2/11/2010 7:30 PM

Both responses contradict each other with adding/removing reverse.  I make my thumb very snug with tape so I cannot bend it.  So is this more likely a mechanical issue rather than a pitch correction?
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- woz

More than likely you need a pitch change.  I battled the exact same issue for a long time.  My old pitches were 1/4 reverse and 5/16 palm.  I tried a ball once with 0/0 and it hurt like mad.  The problem never got better.  After talking to a different ball driller he recommended that I try 0/0 again.  I told him my experience with it.  He told me it's because I was squeezing the heck out of the ball.  I currently use 1/8 reverse and 1/8 palm and the problem is almost gone now.  I'm even thinking about going to a bit of forward because I've gotten so used to NOT grabbing the ball that sometimes I drop it.  

Your driller did the right thing "by the book" when he gave you more palm pitch.  You generally go in the direction of the problem to fix it.  The problem is that everyone is different.  The more palm pitch you have, the more reverse you're going to feel you need in order to get out of the ball.  You'll feel locked in otherwise.
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"You cannot change the stripes of a leopard."
~Emmitt Smith

On the left when the palm is facing down.
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- woz