Hi Welcome to health care magic. Elderly people are quite susceptible to get flu because it is a communicable disease but because you had vaccination few week back, this means you must be very well protected against flu at the time of exposure. It is highly unlikely that you might be having flu but maybe you are having some sort of other viral illness or sort of fatigue state. It is good idea to get rest indoors and Tylenol will help you get some feeling of wellbeing. Meanwhile ensure good hydration and nutrition till you recover. I hope the advice will be helpful for you.