I'm pretty sure that they don't know how long the vaccine will last. I had the same thing happen to me although I was vaccinated much before you. Within a year I showed no detectable antibodies. However, a repeat test showed that I was actually borderline. It has to do with how they calculate your values as compared to a control. If you are not much different, it will appear that you have no antibodies. Next test could show a little bit different and when the calculation is done, you are OK.
First, have another antibody test done if you can. If it's neg, talk to doc about next step.