My period's late...Why?[besides pregnancy]?

My period's usually on time, maybe a few days late if that, but it's late a week now.

Why is that?
I heard traveling messes with it and I've flown a bunch of times the past month and I've been in much higher elevated places than usual. Is that true that travelin affects it?
I've also heard stress can cause it to be late...

How long will it be delayed?
and are there other reasons?
I took a pregnancy test and it came out negative..

depends on your age
during your teen years periods are often irregular in females because of hormones
some people may have a 1-2 week delay
other only a day or two late
some people have extremely irregular periods that they get them almost every day for months (this needs medical attention)
while other girls never get there period and wait months to get it(this also need medical attention)

you may be getting i late for the following
a) your irregular because of hormones
b) a change in your diet
c) high levels of stress
d) a change in exercise and activity
e) you may be pregnant

The main reason you will be late besides pregnancy is due to stress, when your stressed out your body starts reacting to it and that causes your period to become late or abnormal, just try to relax and everything will get back to normal in time, also you might have just tested to early sometimes pregnancy test will come out negative when you are pregnant, if your period doesn;t come soon I would recommend you going to your doctors and having them preform a blood pregnancy test. Good luck!

3 things could be going on

1.Stress, you just have to relax and it will come when you least expect it.

2. Body fat is too low and your body has shut down the female organs, it has to be really low for that to happen.

3. Pregnancy, make sure the test you took wasnt a false negative, they happen sometimes, especially this early.

Stress can definitely cause your period to be late. Check out this article: "Top Ten Reasons Why Your Period is Late"
http://pregnancy.about.com/od/amipregnan...

It's probity STRESS related. If your under a lot of stress this will make you late, reduce your stress level if you can and you should go back to normal

Doublecheck your test. It could be unable to tell at this stage. otherwise, its pressure/stress

stress can delay it or if you changed any of your eating or exercise habits.