I found a pea size lump as well approx 8 months after both a mammo and US. Ended up an aggressive ER PR negative Her2 Positive cancer. I have D density breasts and feel it was not seen by radiology. DO your breast exams and don’t rely on just mammo,especially with dense breasts. If you can afford it do an MRI yearly as well! It’s already in 2 lymph nodes and I’m facing chemo for at least 5 months, then mastectomy and radiation. I’m grateful to my Nurse Practitioner that she took it seriously and sent me for another US and Mammo. ( she survived breast cancer herself). Don’t let anyone say “its just a cyst” get tested!!