Irregular periods..?!?

I started my period when I was 12, and for about 3 years, its been exactly on time every single cycle.. every 28 days. It was super regular. I am now 16 years old, almost 17. But all of a sudden, , for the past 5 or 6 months, its been irregular, and coming a LOT later than it has ever been. Its now around 35-40 days every cycle opposed to when it was 28 days each cycle. Its longer now and I don't know why this is happening. I will add that I have been really stressed out and dealing with a bunch of drama and stuff At school and everything, and I run a lot. I eat normal, and I'm not excessively skinny. I'm about 100 labs. I have mental illnesses and brain chemical imbalances, if that info helps at all.... what could be wrong with me? Should I go to the doctor about it? Please help, thanks!

?2016-05-27T19:47:28Z

Could be that you are a little skinny for your height. Unless you are 5' tall, then you may be more slender than you think, and that can change hormones and result in irregularity. The best person to ask if you are concerned in your care provider.