If pregnancy has been ruled out; there are a number of other reasons for missed period.
These include hormone imbalance, Polycystic Ovary Syndrome, thyroid problems, stress, being underweight, or being excessively overweight, losing weight very quickly or exercising excessively.
In order to get an answer re hormone and thyroid you will need to request blood tests for these from your family doctor. The lifestyle factors can be looked at and changes made at home/work to reduce stress etc.
Your doctor will also want to hear about any other symptoms such as unwanted hair, mood swings, thinning hair, tiredness etc.
Some of these tests can be done with simple blood tests, so you may have had them already however if there is a specific history of stress then time will allow your cycles to return to normal. I would not get too worried about future pregnancies just yet. At you age you still have time on your side.