There are several possibilities. Use the process of elimination to figure out what it could be. Maybe you have a plugged duct in the breast that your baby is dismissing. In this case, you probably have a lump or painful area in that breast. Or, if your baby just got immunized, she could have a sore area on her leg that's being hurt when held in your usual position. Another explanation could be an ear infection. Recent nasal congestion would be a clue that it's a possibility. If this breast refusal continues, you probably want to have the baby checked by her pediatrician. If there is no obvious reason, you may want to see your healthcare provider to examine your breast since sometimes breast disease can lead to refusal of one breast.