That general achy feeling, nausea and low-grade fever that developed soon after you got your flu shot isn't an indication that your attempts at preventing this year's flu failed -- it's actually to be expected. Wait, expected? These are normal side effects in many people, caused by the body's immune system responding to a foreign invader. Additionally, the flu shot may leave your arm feeling sore, and there may or may not be some redness and swelling where the shot was given. All of these side effects should be mild and should only last about a day or two -- not too bad of a trade-off when we're talking about a day with a sore arm compared to a week of full-blown flu.