Hi there. You may not get this reply before you take her to the vets- in which case i apologize for the late reply!
It sounds as if this may either be an abscess- which is a pocket of bacterial infection (which makes pus), or a mammary tumor- which is a growth of the mammary gland tissue.
If it's an abscess, then this would account for the smell, and may be treatable with a course of antibiotics, usually a drug called Baytril.
If it's a tumor, it may need surgical excision to remove the lump.
Glad to hear that she's well in herself, so hopefully some treatment will help.
Best wishes
Andrea
Veterinarian
bvm&s mrcvs 15 years as a companion animal veterinarian