If a ct scan shows a breast lump and the mammogram does not should another test be performed?

Yes: A directed ultrasound of the breast should be done to check the affected breast, as well as a careful physical exam. If these tests are normal, it may be advisable to order an MRI of the breast.

If you can feel it: If you or your physician can feel something abnormal then an ultrasound should be performed. Furthermore, your doctors need to be absolutely certain that the mammogram includes the area of abnormality seen on the ct scan. It is also possible that this "lump" has been present over many mammogram exams and is now considered benign. Ask for your mammo&ct reports and discuss with your doc&radiologist.

Ultrasound: Ultrasound is frequently used to try to find lumps that are not visible on mammography.

Maybe: If the radiologist looks at both studies and determines that a normal structure on the mammogram accounts for what looks like an abnormality on CT, then no. If the mammogram can't account for the CT finding, then yes. Ultrasound would likely be the next test, then possibly MRI if there is still uncertainty.