Bummer...but....first, worrying won't make anything better, but intellectually that's easy to say, emotionally....well....

But more important than that, a callback for additional views and an ultrasound is not unusual particularly for small breasted women. If you didn't have kids, even more so. The breast tissue is dense and hard to get good images of. Given your history and the fact that this is your baseline mammogram the callback should not, in itself, be a cause for concern. At this point, view it as them wanting to get a good baseline.

Best of luck.