Is there any answer for a child to have A negative blood and both parents to have O positive?

I already had tests done, they're my parents. Also, all of my grandparents had O positive blood. My older sister has type O positive and my younger brother has type O negative. I don't get it, and I was wondering if there was any medical explaination as to why my blood type would be what it is. So far there seems to be no way my blood type can be this and my parents to be the recessive allele. I'm really confused, that's why I have all the tests done. Now I'm just angering people trying to get answers, please help!