I had my first mammogram and they called me back for an ultrasound. what could be the reason?

Very common: Ultrasound is often used to evaluate densities seen on a mammogram. Most often a round density on a mammogram is a benign cyst or benign breast tumor called a fibroadenoma. If the ultrasound suggests an alternative diagnosis it can be used to guide biopsy. Basically it is a second test used to provide additional information.

This is a: relatively common scenario. There was a potential abnormality on the mammogram that needed further evaluation. Most of the time, it turns out to be nothing of concern.