Despite the fact I have always kept my cat's vaccinations up to date, my Barty Cat still managed to get cat flu. He started off sneezing lots (I think it's quite cute - he always looks so surprised afterwards!) and then went off his food. I would take your cat to the vets to be honest, Barty's fine now but it did make him very poorly for a couple of weeks, he had to be put on a drip because he couldn't eat!
Apparently, like human flu, there are many different strains and your cat will only get vaccinated against the most common strains.