Non-palpable breast mass has gone from 8mm in feb. '15 to 1.2 cm in june '15. what could it be?

The specific: imaging features determine the probability of malignancy. However, a 50% diameter increase in 4 months would warrant a biopsy even if the imaging features look benign. That doesn't mean it is malignant, it just means that it's time to find out what it is. Presumably it looked benign/probably benign in Feb since no intervention was recommended.