Callbacks for ultrasound and 2nd mammo are pretty common. Like everyone said, it's your baseline so they want to get a good picture of everything and with your history they want to get a REALLY good picture of everything.

When I got my first mammo the tech said I should just assume I'd be coming back in for a 2nd and/or an ultrasound due to my cancer history and the breast cancer all through my family. Luckily my floppy saggy deflated "girls" squished real darn good between those plexiglas sheets! No need for a repeat.

Call the doc and tell them you're concerned, let them tell you what's going on or maybe move you up sooner if another appointment opened up.