' Wouldn't there be an obvious lump if something were wrong?'
No, not necessarily. I would also go with your doctor's hunch and do the biopsy. There is a reason she is concerned, and your doctor is not only trying to cover her behind with you, as in.....afraid of lawsuit if she happens to be wrong even if there is no obvious reason to preoceed with an invasive procedure.
She sounds good and it also sounds like you trust her.
Katrin
PS: kumps are not always seen on scans and/or felt by palpitation.
Although it is YOUR decision I personally would go ahead with the recommended biopsy. Watching rarely solves anything and certainly NOT a question as to what the Dr.s are feeling in your breast. Many things can on occasion be missed by both Mammogram and Ultrasound. In the end a biopsy and ONLY a biopsy can determine with 100% certainty the true nature of some abnormality within the breast tissue. All these things you state (?family history, one child, didn't breast feed) really have very little to do with what may be present now. Regards ....