I agree with everything that cmpgirl said here, and want to add a couple things.
First, I think it's good that at 17 you are so in-tune with your body & your menstrual cycle. Many young women just don't pay any attention to these things. I have always kept track of mine for some reason and I started when I was 11 years old.
Now, like cmpgirl said, stress can cause this to happen, so if you're under lots of stress, this could easily be the culprit.
Other things that can cause this (besides pregnancy) is weight loss. Have you dropped weight (especially quickly) recently? Or, have you become active in sports and dropped body fat without actually really losing much weight?
A female's body fat also aides in production of estrogen, so if you are thin, your estrogen levels may not be sufficient to cause your period to come regularly.
My daughter has always been thin (except as a baby) and never actually began a normal and regular menstrual cycle. I mean, she maybe had 2 periods a year. We had to put her on birth control to "kick start" her periods. She is almost 19 and since starting the pill, she has regular cycles. I'm not saying this is what will be right for you, but these are some things to think about.
This could also be a simle issue of some kind of vitamin defficiency too. There are so many reasons a woman's cycle can be interupted.
And, like cmpgirl said, try not to worry too much about this. If you continue to have this problem, you GYN can always steer you in the right direction.
You can always call your GYN's office and tell them what's going on. They may want you to come in for a pap smear. Has it been a while since you've had one? If so, then go ahead and schedule one. This is another very important part of female health. I have to add that any time you have a change in what is "normal" body function issues, it's generally a good rule of thumb to speak with your doctor if it doesn't clear up in a fairly short amount of time.