Some breast lumps feel as though they have a distinct border, while others may feel like a general area of thickened tissue. What Causes Breast Lump? They account for 50 percent of breast biopsies, but they do not usually become cancerous. If you notice a lump while you're breastfeeding , don't ignore it. An ultrasound is a noninvasive, painless procedure that uses sound waves to produce images of your breast.