I had a callback after my mammo last year (my 3rd so far). I went back in for a diagnostic mammo & ultrasound. Turns out it was only a harmless cyst. They suggested I come back in 6 months for another ultrasound (which I did), and it had decreased in size by then, so everything is fine now.

Try not to stress out too much over it (I realize it's hard not to). It's very common to be called back and it will give you peace of mind.