My son does the same thing all the time, and my daughter did it too. I think what is going on is that after the milk has let down and the “easy” eating is over, and they have to start sucking a little harder to get the milk, they start refluxing, and it burns or hurts… switching to the other side, to a full breast, means that the milk is once again plentiful and it washes away the burn. That’s the only explanation I’ve come up with. There are times though when I can’t get him to nurse on one side hardly at all, but he will on the other. Maybe it’s the angle or something??