A pregnancy test taken in the first couple of weeks of pregnancy may not be able to detect the rising hormone levels. So I would be inclined to do a second test, or ask your doctor to take a blood sample as this is more accurate.
Assuming pregnancy has definately been ruled out, there are a number of other reasons for late or missed period.
These include hormone imbalance, Polycystic Ovary Syndrome, thyroid problems, stress, being underweight, 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.
Once the cause has been established then hormone or other supplements may return your cycle to normal. Depending on how late you are, you may still have a period and it may be heavier than usual. This would indicate changes to your hormone levels.
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