My freind had a similar experience 9 years ago. She had recently given birth and was beast feeding her new baby daughter. Her doctor told her not to worry about it. He said it was probably a blocked milk duct. Shortly thereafter (not sure how long -- 3, 6 12 months), she was diagnosed with breast cancer that had spread to other body parts. She was a fighter. She died 13 months ago after 8 years of aggressive life changing cancer treatment. I'd suggest you see a qualified doctor as soon as possible.
It is possible to develop a malignancy while pregnant or breast feeding. Please check with your local doctor and they can help you get evaluated with a high resolution ultrasound or a mammogram and these will not affect your breast milk or feeding. Make sure you follow up on this immediately. The costs for these tests vary and are usually covered by insurance.
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