Can your blood type change?

On my birth certificate it states that my blood type is A+. Yet when I donated blood in 2007, I received a my Red Cross blood donation card and the card stated I have O+ as my blood type. How is that possible? Did some one make a mistake at the hospital or Red Cross? Did my blood type really change? I know the chances that the hospital made a mistake is slim, due to the fact I was born at home. So I was kept separate from the other babies and kept in a room by myself. Which I think should of reduced the chance of them mixing my blood samples up with another baby's blood samples. I also know that my mother's blood type is O+, while my father's blood type is A+. I have also never received any blood transfusions through out my 21 years of life. I want to know for sure is it possible for my blood type to change from when you are born to adulthood, so I know whether I need to have my medical records and birth certificate change or to just get a new Red Cross blood donation card.