RH- Confusion!!!!

I am pregnant with my 5th child and my doctor tells me I am RH- well here is the issue the other 4 it never showed the hospital and other doctors office shows that I am A+....I know blood type don't change and they have drawn blood twice to double check but how can you go from A+ to A- with the -RH factor? Idk what to think...and I have never had the Rogam shot with any of my other children which makes it more weird....