I rarely get this callous during casual play, and normally only after a 2-day, 4-round tournament, as I had this weekend. I don't think this is a normal callus and it makes me wonder if I have a grip/release problem. I don't get any noticeable flutter when I throw.
It's on my palm, in between my life lines and the disc rests on top of it (ie. the disc is right into that crease). I have two other normal callouses, one at the very bottom of my index finger that you can make out in the picture, and another on the outside of my index finger up at the last joint.