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!