Mastitis is usually the result of a blocked milk duct that hasn't cleared. Some of the milk banked up behind the blocked duct can be forced into nearby breast tissue, causing the tissue to become inflamed. The inflammation is called mastitis.
It happens in about 10% of breastfeeding women. It’s also common amongst cows and goats! I don’t know why it’s so common but it’s not rare and it really sucks!!!