Welcome to HCM.
You have not mentioned whether she has regular periods or it is usual for her to have delayed periods.
Since you are taking proper protection precautions and since the pregnancy
tests have been Negative, it is unlikely she may be pregnant, but just because the test is Negative, one cannot be 100 % sure that she is not pregnant, because pregnancy test only indicates that the pregnancy is not of that duration which can be detected by that test.
Even mental stress
can prolong periods.
Try not to have sexual intercourse till she gets her period.