Why A Pregnancy Test?

I went to the doctor bc I'm going to be analyzed/diagnosed with something using a test that uses some type of radiation. I told her I was not sexually active when asked. She then asked me if I'm pregnant. I said no (not sexually active). Today I got a call from a nurse to help me choose an appt date. She asked me if I'm pregnant. I said no. She said I was going to have to take a pregnant test at my appointment. Wth! Anyone know why?