Hi and thanks for the query,
the commonest cause of late or delayed periods is a pregnancy
. You should be sure of the quality of the pregnancy test you are doing, it might be indicated to maybe do a blood Beta HCG
test in a medical setting which is more sensitive.
It also common amongst women of reproductive age to have some irregular cycles, certainly not because they are pregnant. It is therefore not surprising that the pregnancy test you did could also be actually negative. These could simply be delayed periods.
I suggest you consult your primary care
physician for an appropriate pregnancy test to be prescribed. Waiting for a few days again could do, in case its just a delayed period, the menses might just be coming very soon.
Thanks and hope this helps, best regards.