My radiologist ultrasound -- bi-rad 3 possibly benign for septated cyst on left breast 1.4cm x 2.8 cm, no internal flow, no solid nodule..is it really benign? why the word possibly benign? last day period felt pain/lump, 2days after no pain just lump

Histology: Radiology is not the definitive way to make a diagnosis of a breast mass/cyst. The radiologist is using the term, "possibly benign" out of caution. You should consult with your doctor to ascertain if the lesion needs to be removed and sent to a pathologist for definitive evaluation. Wish you good health!

Do you mean: "Probably Benign"? That is a standard(and federally required for mammo under the Mammography Quality Standards Act) category of lesion classification based on various imaging features. The classification is called BIRADS and category 3 is "probably benign"(1=neg,2=benign,4=suspicious,5=highly suggestive of mal). A "3" implies <3% chance of cancer, short interval follow up is the usual management