Check LATCH. It could be that your baby has an incorrect latch or his/her mouth is not open wide enough when feeding. See a lactation consultant or pediatrician to evaluate both the latch and your breast.
Could be infection. Redness on the breast particularly in a wedge or pie shape could be a sign of infection. This is especially true if the red area is warm to the touch and somewhat tender. See a provider who can determine if you need antibiotics.