Radiologist says possibly benign from ultrasound septated left breast cyst 1.4cm x 2.8cm. no solid nodule and no internal flow. is this serious?/bad

"Probably benign": is the BIRADS classification(category 3)you are referring to. By definition, this implies a chance of malignancy less than 3%, and practically speaking it is probably more like 1%. The standard recommendation for a category 3 lesion is short interval follow up. Many consider a septated cyst a BIRADS 2(benign) lesion requiring no follow up, so the overwhelming probability is that you will be fine