I have seen this twice. For one of my girls, it was mastitis. Treated with compresses and antibiotics. The other girl it was mammary cancer. Interestingly, the girl who had the mammary cancer had a teat that was inverted and had mastitis in that teat for both litters. We removed the affected teat and the one on each side. She recovered nicely from the mammary surgery however cancer had spread to her lungs and we put her down 9wks after her surgery.
