Hey Toddster,
I've had that problem years ago. Sometimes you can be placing too much pressure on the arrow nock with your index finger. Two things, is your string angle pinching your fingers, or can you just place your finger about 1/8" above the nock when you grasp the string? Also, I nock above and don't have that problem anymore. Those are a few things you may check out.
Some people also get porpoising arrows, so I know getting that finger above the nock a touch can help assuming you nock your arrow below the string nock.