Despite the longstanding myth, you can’t get the flu from the flu shot. Since the flu shot is an inactivated version of the virus, there’s no way that you could actually contract the flu from the vaccine (via Health). It is not unusual for people to feel mild flu-like symptoms within a few hours of receiving the vaccine. Side effects of flu shots include nausea, headaches and muscle pain.
These side effects are milder than the flu virus and last for a shorter time. These symptoms typically disappear within 24 to 48 hour of getting the vaccine. You should consult your doctor if they persist for more than 24 hours. It takes two weeks for the virus to fully protect you. This could mean that you may have caught the flu before you were vaccinated.