Late period, is something wrong??

 I haven't had a period for the past six months. I'm 16 years old and I used to have pretty regular periods until this happened. I was training for a half marathon and I thought all of the running was slowing my cycle but that was three months ago. I still havnt had any signs of my period coming. I know I'm not pregnant because I've only had sex once in the past six months. If I was pregnant I'd have a pretty big belly by now! I'm really confused about what's going on. Help if you know what it could be. Should I go see a doctor?