Milk blebs form when a duct gets clogged in your breast, resulting in a thickening of breast milk and a slower flow near your nipple's opening. A milk blister may also develop when a bit of your skin grows over the opening of a milk duct, causing milk to back up in the breast. Causes of the blister itself can vary and include: baby latching, tongue, or sucking problems excess milk supply excess pressure on a particular area of the breast thrush, a type of yeast.