Hello, I am Dr. Peter a licensed Veterinarian. I would like to help. I need to ask you a few questions to be well informed and give you the best advise. There is no additional charge for information request.
1- Is she spayed?
2- Is the swelling limited to the nipple or is the gland (below the nipple) swollen as well?
no she is not spayed
yes the gland is swollen
In response to your question, this type of swelling of the mammary glands in dogs is a concern because it could be a tumor; 50% of mammary gland tumors in dogs are malignant. Your dog needs to have either a biopsy or fine needle aspirate to evaluate and determine if it is a tumor and if so, what type. Surgical removal, +/- chemotherapy is the recommendation if it is malignant; consider having this checked as soon as you can; the sooner we have a diagnosis the better the prognosis.
If you have additional questions I will be glad to answer them.