Q. Hi my dog breast is swollen what i do
DESTINI R. HOLLOWAY, DVM
A. If your pet is not spayed or was spayed after 1-2 years of age there is a chance that this swelling could be mammary cancer. Mammary cancer in pets is common and the chances of cancer are decreased an the risk of getting cancer is only 0.5% if spayed before the 1st heat cycle, 8% if spayed before the 2nd heat cycle, 26% if spayed by the 3rd heat cycle. After the 3rd heat cycle spaying a pet will not decreased chances of mammary cancer.
I would highly advise that you take your pet tot he vet to have this area examined. Typically mammary cancer lumps are firm and can be on multiple breasts.
Without seeing the lump myself, another possibility here is a lipoma (soft fatty tumor) or an abscess.