Normally they tell you on the phone why they want you to come back. Obviously, something does not look right and they may want to do an ultrasound.
Laura
Try not to be alarmed. I get called back every year for an ultrasound. It just means they want a closer look. I just went back again today. They found a cyst. It may go away on it's own or I may have it drained. Just try not to worry. Many people get called back and alot of us have fibrous dense tissue. Good luck!