Dear Elaine...I have absolutely no experience with US, so I can't answer to that. However, at 36 I was diagnosed with two different types of invasive breast cancer, both palpable lumps, neither of which showed up on a mammogram. My mammo read as dense breast tissue, with no evidence of areas of suspicion. I recieved a post card declaring my mammogram results were "normal" two weeks after biopsies were confirmed for invasive breast cancers. The younger you are, the more likely a mammo is to miss something. At the time I was diagnosed, about 30% of all breast cancers were missed on a mammogram.
I think ultrasound, when used for a palpable lump that a mammo has not shown, has reduced those percentages drastically. And MRI's are proving to be useful diagnostic tools, too.
I had the "fibrocystic breast" stuff, too, and yet I knew that the lumps I found were different things entirely.
What is your doctor telling you? Are you going to have the lump biopsied? That is the ultimate guarantee that this is "nothing." And sometimes the peace of mind that comes with a negative biopsy far outweighs the little scar it leaves. I had another lump on my remaining breast a couple of years after my first round of brca. A mammo showed nothing. My surgeon thought it was nothing, too. But he immediately scheduled a biopsy for me anyway, because, in his words: "I guess you've heard that before, though, haven't you." This time, he was right, I was wrong. Thank goodness! A little scar is a small price to pay for the peace of mind that a biopsy can give you. So don't be afraid to insist on a biopsy if that is the only thing that will let you rest at night.
"There's a difference between a philosophy and a bumper sticker." --Charles Schulz