Welcome to HCM.Pregnancy
is only possible if unprotected sex is done around ovulation time in regular period.
You did sex with using condom and you get period 16 days late with negative pregnancy test.
So possibility of pregnancy is almost nil.
There are several reasons for delayed period like psychological upset,altered eating habit,wt loss or gain, hormonal
,excessive exercise,ovarian cyst
So consult with your doctor and need some investigations to evaluate underlying pathology including hormonal study.
Avoid stress by regular deep breathing,take healthy diet and proper sleep.