You very well could be pregnant. I would take another test. If that one comes up negative, then i wouldn't worry about it. A lot of things can interfear with your cycle and that may be what is happening. If you are really worried, call your doctor.
If you don't want to take birth control or use other methods of pregnancy prevention, look into charting your cycle by taking your basel temperature. That way you will know when you are most fertile and likely to get pregnant or not get pregnant. Good luck