Feeling a little sick or 'under the weather' after a vaccine is a normal reaction. Although the vaccine does not contain active viruses, your body is working overtime to learn how to prevent the infection, which can cause some people a sense of unwellness. Additionally, some individuals react adversely to the stress of vaccination, and it's possible to be allergic to one or more ingredients in your shot. Overall, you shouldn't be concerned about feeling sick after a vaccine, but if the symptoms persist or are more severe than usual, you may want to make an appointment to see your primary care doctor.