Two yrs ago, the same thing happened to me. I was called back for a 2nd mammo and ultrasound. After that, an appt was made for a breast MRI. It is only natural to be frightened, and while yes, it alarmed me some, I was determined to stay calm until there was really something to worry about. Turned out, there wasn't. It was all due to dense breast tissue.
Let us know the outcome. Good luck!
I recently had a callback, and it was because a saline cyst (visible in my mammogram a year ago) had gotten smaller. Don't borrow trouble, there is little enough point in worrying until you have something to worry about. :)