I'm 38 and and use no contraception. I've had 3 children and lately when my period is due I get light headed and a bit nauseous. Is it normal for this to happen and can your symptoms change as you get older?

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 - Floradix is a good product to look at.

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) and do come back to me should you have any queries or questions.