Sounds like a little OALD to me. My DD will do that too, sometimes when she is gulping the milk down (I can hear the gulping and it's like hurts going down sometimes). Nursing reclined and side-lying had helped a little. Otherwise, I just try to calm her and try again a little later. She is 6 weeks old now and it's starting to get a little better - probably a combination of her coping better with it and my supply/let-down calming down a little. Hang in there. I know it's hard to see your LO cry during feeds. But keep up the good work and it will get better!