It depends on what you mean by feeling "sick"..do you mean nausea? Periods can be very painful, your body temperature naturally rises and your hormone levels change dramatically. If you have heavy periods too then you can also feel very drained and lightheaded. You are losing a lot of B12 during a period so taking a supplements, starting the week before your due, may help with your symptoms. Most of us feel 'yukky' during our periods and its quite normal to feel this way. I find painkillers and chocolate help me through it lol!