Why would a radiologist have the hospital call me 24 hours after my mammogram to request that i come in for an ultrasound asap?

Abnormal mammogram: Most likely there is something in your mammogram that needs more evaluation and ultrasound is needed to confirm and evaluate further on the abnormality in mammogram. Call your ordering physician as most likely your doctor is the one who ordered the ultrasound as well.

Suspicious finding: On second thought radiologist felt that more information was required. Could be a small cyst or what could be small fibroma or tumor. Ultrasound is next step in checking it out. Odds ar in your favor that it is benign. Still in your favor if ultrasound suggests need for a biopsy. Worst case scenario, which is unlikely but pupose of mammogram , is that you have a small/early tumor. Good luck.

The radiologist: saw something that needs further characterization, possibly a mass or potential cyst. Usually these turn out to be nothing of concern, but sometimes not.