I got the same call a few months ago and was terrified!!! It turned out that it was a false positive, i.e. there was tissue overlap in the left breast and the machine picked it up as a shadow. These false positives are very common, especially if you are under 40 and the breast tissue is quite dense.
My referral slip also had ***** w/ ultrasound. They did the ***** first (spot compressions, kind of painful...) and when they saw that the suspicious area spread out with localised pressure (ouch!), they skipped the ultrasound and asked me to come back in a year or two.
Try to relax, there is a very good chance that you are being called back because of a false positive. Do let us know how things turn out.