J.S.
It might be latch due to his teeth starting, or it could be thrush. Below is a list of symptoms from BabyCenter.com. Check in his mouth and see if he has any white patches as well. Maybe that's your culprit?
"Some common signs of a yeast infection in nursing mothers are:
itchy, pink, red, shiny, or burning nipples (may be cracked) (Note that pink, tender nipples can also be a sign of a bacterial infection, or dermatitis, which should be diagnosed and treated by a dermatologist.)
deep, shooting breast pain during or after feedings
a vaginal yeast infection"