The flu vaccine is very hit or miss as there are multiple strains each year and vaccines are only developed for the strains most likely to be the most active each year so there are many possible explanations for your case, but one thing is certain - the vaccine did not cause you to get the flu as a dead/weakened virus cannot cause the illness. Also, you may have had the flu before but not recognized it so I'm not sure you could say you've never had it before except that one time unless you've done extensive blood testing. But let's say you really never did have the flu except that one time, well, if millions of people are tossing coins you would expect that there will be cases where for some people it always comes up heads with one tail.
