Why my blood type is different from my parents.

When I was pregnant, I found out that I had B positive blood.  Both my parents are O positive, and so is my brother.
I know my parents are my parents because I look like both of them, and I have their same genetic health problems. My mother's mother had B positive blood, and maybe it skipped to me?  What do you think?
Thank you,
lind28

[eureka254]

You might find these links helpful:

http://www.biology.arizona.edu/human_bio/problem_sets/blood_types/Intro.html

http://anthro.palomar.edu/blood/ABO_system.htm