B.W.
I've been having this in my right breast since my son was about 6 weeks old (he's 11 weeks now). I also have a pretty high pain tolerance (3 unmedicated vaginal births...) and this pain would send me to the medicine cabinet almost every time! My doctor explained that it is a spasm of the breast muscles upon emptying. Since my little guy is a strong little sucker, he can empty my breast VERY quickly. What I did to remedy this is to only allow him to eat on that side for a maximum of 10 minutes. If he still seems hungry, I will offer him the other side, which he ususally only eats for another couple of minutes before he is done - then I will start with that side for the next feeding. DON'T STOP BREASTFEEDING!!! =c)
Good luck -