I think that when you become ill, this can delay your period by up to a week at least. If you get flu, your body has to deal with this and try to fight it, so if your period is due it may be delayed, as your body has more important things to deal with other than having a period. Once you are feeling better and are up and around again, I am sure your period will come, unless of course, you are pregnant.
This has happened to me before now and it seems that it is just a natural response and the body trying to get its priorities right.
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flu and menstruation 
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