What Causes Delayed Periods?

Ok, i'm 14 and my period was due over a week ago, and it never came. I cant be pregnant becasue i've never kissed a boy, let alone had sex. I've had my period for about 7 months and this is the first time it's been late. There has been droplets of blood twice but only a drop and nothing else. I'm really worried and im going swimming tommorow and dont no what to do! please help,