I had my flu jab earlier in the year and now I've got flu. Why?

This is not an uncommon experience. The key reason is that the flu jab only protects you from a few types of flu viruses and it is likely that you have picked up a virus that you were not vaccinated for.
 
In addition, sometimes a bad bout of the common cold can seem like flu and of course, the flu jab does not protect you against the 100 types of common cold viruses lurking around.
 
Please read my blog - Why you can still catch a cold or flu after the flu jab - for more information.