You could have possible had a miscarriage. Sometimes there is something genetically wrong with the chromosomes and the result is an early pregnancy abortion. Most women have 1 or 2 miscarriages in their life and don't even know it! So there is a possibility that you were pregnant but your body has now flushed it out. But don't fret over it.... There's nothing you can do to prevent a miscarriage from happening. If your period does not return to normal please contact your doctor to get expert medical advise. Good Luck!
Source: Lost first baby at 12 weeks and am now 21 weeks 4 days pregnant
Source: Lost first baby at 12 weeks and am now 21 weeks 4 days pregnant