I had my first flu shot a few days ago and have felt lousy ever since. Why?
Unpleasant reactions to flu shots are quite common. Basically, you have been exposed to a foreign viral substance similar to that you are exposed to when you get the real flu. You feel ill because your immune system is having a reaction to this exposure. Presumably, when you are exposed to the real influenza virus, you won’t have much of a reaction because your system has acquired an immunity.
I generally don’t recommend flu shots unless a person is quite elderly, has fragile health or is immunocompromised (i.e., has HIV or any other condition that suppresses the body’s natural ability to fight infection). For the record, you might want to report your reaction to your public health department.