Change in blood type?

Neenah
I went for my 28 week check up this morning. I did the one hour glucose test as well got my blood draw for testing so I can receive the Rhogam shot. With my other two pregnancies I have been O- and have gotten the shot with no problems or complications. My midwife called me after the appointment not only to tell me that my blood sugar was elevated but my blood type was showing to be O+. I never had gestational diabetes before and never received the Rhogam shot this pregnancy so how is it that my blood mysteriously change from O- to O+? I thought she was reading the wrong chart or maybe there was a mix up in the lab testing. Had anyone else had this happen to them before?