Wow. Almost exactly my situation. In January my breast started aching in one spot on upper outer part of breast. My doctor nor I could feel a lump. I then got a high definition ultrasound that saw in that exact location a benign cyst so my doctor said most likely a cyst is causing the aching.

Two weeks later, in February, I had my yearly 3D mammogram and it showed nothing wrong, ‘all normal with no cysts, masses or malignancy found’ and return in one year. My doctor said the cyst from two weeks earlier on ultrasound most likely dissolved.
BUT my breast still ached. By May I insisted my Dr order a MRI. The MRI showed a highly suspicious solid mass with irregular borders in EXACT location the ultrasound on Feb 2nd called a benign cyst.

A biopsy proved that so called cyst, and what 3D mammogram totally missed, was a cancerous tumor. Unreal. I HAVE LOST FAITH IN THE ULTRASOUNDS AND MAMMOGRAMS.