Is it strange that I am so late?

I'm a teenager, and my first period was bit over 3.5 years ago. I read that by now, my period should be regular, however, I've noticed that recently, my periods have not quite been. At first, as expected, they were irregular, sometimes less than a month apart, usually a bit over. Then it steadied a bit, and were usually 40 days or so apart, but this only lasted for a few periods. Last time, my periods were over 50 days apart, and now it's been 72 days since my last period. I'm certainly not pregnant (I'm a virgin, haven't even kissed a guy yet actually), and I realize I'll probably get my period soon, but I'm just confused as to why it's so late right now... I'm 17.