Okay well there are many reasons for missing a period.
For example if you have only recently started your period then the time gaps between them can vary a lot, this is perfectly normal.
Another cause for missing a period is stress, if you stress you put stress on all of your body this sometimes causes you to miss a period, they should return to normal when you do.
If you are still worried about being pregnant if you go to your GP and get a blood test that will determine whether you are pregnant or not.
Good Luck