Lots of stuff could be causing it. Here are a few that I can think of. Are you using a metal nock set? If so, going to a tie on thread nock set will help. One way or another, you're getting too much pressure on that finger. You could be exerting too much pressure yourself. Could try relaxing it a little as you draw. Is your drawing arm elbow high? That tends to put downward pressure on the index finger. In that case, lowering the elbow should help. If you have a fairly long draw and a fairly short bow, the angle of the string will tend to push your fingers toward the arrow (finger pinch). A glove would help with the symptoms, but likely won't clear up the problem. Hope some of this helps. Good luck.