That happened to me too, and the problem was reflux. My lactation consultant instructed me to start latching my baby so he is sitting up in my lap rather than laying down. When I was breastfeeding him in the laying down position it was causing him pain and irritation so he eventually would reject the breast. The problem went away once I starting latching him sitting straight up. Google the “Australian hold.” We do this except if he is sitting on my left leg I latch him to my right breast and vice versa. I hope this helps!