OMG I just found this thread.

Turns out, 2 years later I found out it *was* breast cancer. At the time I had 2 lumpy structures, one of them was a blood vessel (as above) the other grew to a tumour measuring >10 cm. It was disguised while I was breastfeeding and the lump only became more apparent after I weaned DD2 at 16 mths.

An ultrasound alone is not enough information to rule out breast cancer unless a specific and identifiable structure (like a cyst) can be found to explain the lumpiness. I should have insisted on a fine needle biopsy or a mammogram as well. I am typical of many young women whose breast cancer goes undiagnosed because we are told our young breasts are "too dense" to make a mammogram useful.

The delay in diagnosis cost me my breast, the lump was so large I needed a mastectomy. My surgeon told me just last week how freakishly lucky I was that it was picked up when it was, since it was growing very fast and on the turn to being a very large nasty invasive cancer.