Urine
pregnancy test is most accurate when done early in the morning, the first voiding sample. That's the time when urine is most concentrated and has adequate levels of HCG hormone( pregnancy hormone) that shows up in the test.
More definitive method is to measure serum
beta HCG levels. If that is in the 'pregnancy' range, you are
pregnant.
Else it could just be delayed menses.
If the blood hormone level is also negative, you can get tested to see if there is any other cause for delayed menses, apart from pregnancy.
Next Steps
Serum
beta HCG levels (fasting)
Health Tips
If above is negative,
Get Setum
TSH levels done, to rule out hypothyroidism. You might also require evaluation for PCOD.
BUT YOU CAN RELAX IF THIS HAS HAPPENED ONLY ONCE.