isn't the general rule with women and hormones and stress that anything can happen?
If you are worried, have a pregnancy test because why wait a few days worrying about it - do it now and if you get your period the next day does it matter? because otherwise it will stress you out waiting for your period.

besides stress there are many other things that can cause menstrual probs and what is normal depends on your age as well. go to your dr if you are concerned. if you have any other symptoms you should really go to the dr. I would also be worried if you have been bleeding every 2.5 weeks, are you becoming anemic.
personally i'd go to the dr regardless because i would find menstruating every 2 weeks very annoying and i wouldn't put up with it.

hope everything works out ok