It is normal for any bitch 2 months or so after a season to show signs of false pregnancy which include enlargement of the breasts and milk production. Any change other than this in breast tissue can be scary and is worth a visit to the Veterinarian.
Mammary growths / Cancer usually show as firm nodules in any of the breasts. These can be benign or malignant. In cats where they are more rare they are unfortunately more often malignant. Whatever they are they are best parted from their host as soon as possible and we would generally want to do early surgery on any breast lumps and want to send the lump for a pathologistʼs opinion.
Generally speaking make a fuss about breast lumps.
Mammary cancer is one of the chief reasons why you should have your bitch spayed. If done early before a bitch has a second season the chances of breast growths are very greatly reduced.