well ive been having my period for about 4 years now. i got it when i was 13 and im now 17. well at first it was all crazy and i understood that when it starts your body needs to get used to the changes but then it finally regulated to 28 days. but slowly the days started shortening and now i get it every 21 days instead. Is this normal? Its been like this for id say about a year or 2. Ive always wondered if its normal. Thanks :-D