Ooooh Jett used to do this!!! Man it was soo annoying. He only started doing it around 8 months or so, and he hurt them so much I had to use nipple cream to heal them as they got so cracked and sore! Yet I didn't need any cream when he was born!
Sorry, I don't have any advice, just letting you know my son did the same thing, but he did stop doing it after a while. Although when he would do it, I would take him off the boob, put him down and sternly tell him off, it would hurt me soo bad!