Period is 3 months late? uhhhh?

I'm 16, my period is usually regular. I'm a virgin, so i'm not pregnant. I've had my period for like 5 years and now out of the blue I missed 3 months? I haven't changed my diet, i'm not stressed, I haven't been working out more or less then usual, so what happened? Is this bad? Should I see a doctor? (I'm 5'9 and I weigh 136lbs)