You absolutely cannot get the flu from the flu shot. This is because the virus is killed and not just weakened. The most common side effect of flu shots in adults is soreness at the site which lasts about two days. Rarely, some people will get fever, muscle pain, and feelings of discomfort or weakness. If you feel sick after a flu shot it could be that you’ve been exposed to the flu before you got the shot because it usually takes a couple of weeks for your body to build immunity.