in the reproductive cycle of the woman, there usually exist some cycles called anovulatory cycles. This will be characterized by going through a cycle without bleeding (no menses). This could be a normal possibility in your case.
However, in clinical practice, the commonest cause of lack of period or menses is pregnancy
. Its true you did a pregnancy test. It shall depend on the pregnancy test used and the quality of the technique that was used.
It could be worth interesting repeating this test again or using blood samples. If its negative, this excludes a pregnancy and could possibly be an anovulatury cycle.
thanks and hope this helps,
Thanks and best regards,