Biting or clampdown reflex
I had the most awful/painful beginnings of breastfeeding with daughter number 3. She was born too quickly (17 minutes, stage 2 labour) and this caused her skull to not move as it would if she were born less quickly. It resulted in her biting till my nipples were bleeding and painful (toe-curling, nail-biting, tearful pain). Her fontinels were very small & skull/bones were too tight and was causing her jaw to clamp down. Took her to the chiropractor and the problem was fixed over-night in one gentle, easy visit. Thank goodness!