Why are my dogs mammary glands swollen? Asked By: Constantin Toy. Date created: Sun, May 9, 2021 1:04 AM. Best answers. Another reason for mammary enlargement is mastitis, an infection of the mammary glands. This is painful, the milk is usually blood-tinged, and the bitch is clearly sick. This is treated with antibiotics and hot packs. In older dogs, breast cancer must be considered if a mammary lump is detected. Answered By: Destiny Crona. Date created: Mon, May 10, 2021 3:07 AM. FAQ. =I ...