"Nipple soreness" refers to a discomfort or pain that occurs in the nipples, the small rounded raised parts on the chest, typically experienced by women during breastfeeding or due to other reasons like hormonal changes or friction. Full definition
Poor positioning and latch - on is the most common cause of nipple soreness in the early weeks of nursing. (motherandchildhealth.com)
This position keeps him from having to turn his head to reach your breast; thus making swallowing more difficult and perhaps causing nipple soreness. (motherandchildhealth.com)
Please keep in mind: some women do experience nipple soreness the first few days of breastfeeding. (myorganicformula.com)