It could be stress and the change in your body of course! All those things help to delay periods. Also, you are 16 and for young women, missing a period is quite normal. When I was your age I went through the same thing right up until I was about 18 and I also got my period at age 10. My doctor told me it was quite natural and that my body was just trying to find its rhythm and a system that works. I also had lengthy cycles but now I have my period every 28-30 days. Give it time.
I would recommend going to see a gyno though, just to ensure that you are good and healthy. It could also be a simple hormone imbalance that can be fixed easily.
Peace and Love :-)
Anna