How old is ur baby? Most probably it is due to latching issues. I had this issue, so the baby was not suckling milk. Its best to correct it immd. I kept inserting my finger at the side of the mouth everytime I noticed wrong latching by my kid. My nipples got sore, but I endure it. Cause end of the day, when the latching is correct, u won't feel pain or sore and baby gets the milk it needs.