No way. First, consider whether the lump could be a plugged duct. Treat it as a plugged duct for a week. If it doesn't go away, have your doctor take a look (and don't freak — most breast lumps are benign). You won't need to stop breastfeeding for a mammogram, breast ultrasound, or even a biopsy. However, you'll need to make sure all of the medical folks you deal with know you're breastfeeding. The radiologist who reads a mammogram or ultrasound should be experienced in reading results from breastfeeding women, and biopsy surgeons should be asked to avoid damaging milk ducts if possible.