Your period being different could be the result of any number of other factors. Have you made any changes in your diet, or are you doing [more] exercise? Also, if you've been stressing about the possibility of pregnancy, that could definitely make you get it early - stress does weird things to you.
If you can get your hands on a pregnancy test, then definitely take advantage of it if you're suspicious of pregnancy. I know, it's easier said than done. You're 17, so you're still a minor. If there is a planned parenthood clinic in or around your area, you could go there and they would do a test for you for free.