Blood Type Changes: Understanding Fluctuations and Testing Inconsistencies

Question
I was wondering if it is possible for someones blood type to change?  I have been told that my blood type is A neg since 1993.  I have had surgeries and all during this time.  When I got pregnant,2001, I was even told I was RH neg.  Then when I went to get my Rhogam shot, 2002, they tested my blood (3 times) and it came out A pos.  I just recently, 2004, had it tested again and it came out A pos.  How is this possible?

Answer
While this sounds complicated sometime in your pregnancy you became sensitized to Rh+ blood most likely from the baby's circulation in the placenta. It is also possible that the Rhogam shot in prior pregnancy(s) if you received it did not completely protect you from sensitization. OK?