Missed Periods

Joy
I'm 17 and have had my period since 13. I was just wondering if anyone else has missed periods/ has an unregulated cycle? I'm currently late for mine but this would be the third time that I haven't had one and 
I haven't had sex so I can't be pregnant. Went to my doctor and she said that it could be because I'm stressed. (Also change of diet maybe?) Has anyone else had this experience? TIA. 😘