It is indeed quite normal for your monthly symptoms to change as you get older.
It's possible that if you are light headed before your period that your blood pressure may be low; have you had this checked out?
It is also possible that it's anemia (due to heavy menstruation) and if that is the case then an iron supplement may help.
The nausea may be caused by low blood pressure, heavy periods or even a sluggish circulation.
My advise would be to have your blood pressure checked (if you've not already).