Confused on blood tests?

I am currently 10 weeks 1 day pregnant. I just got my blood test results back from my OBGYN. Everything looked perfectly normal except one thing. When I was little, I had a shot for rubella. It is listed on my shot record and my mom remembers me getting it. When my blood test came back, it said I was negative for being immuned to rubella. I asked the doctor what that meant and he said that usually means I never had the shot for it. How could that be possible? Of course it has been 25 years since I would have got the shot, but shouldn't it still be in my system? Rubella can cause a lot of damage to an unborn child. It is very rare, but can happen. My doctor told me to stay away from large crowded markets especially in the next few months. Anyone have a clue of why it is not in my system?