Zaaaraahh, have you been under a lot of stress with school and stuff? A lot of things can affect ovulation (likes stress, illness, lack of sleep, change in diet, whatever) and that can really throw off your period.
If you've got a negative test, try taking another one and popping into the doctor for a blood test. If all of that is negative, stop worrying and be more careful next time.
While you're at the doctor please discuss more reliable forms of birth control, there's a lot out there!
Good luck!