Welcome to HCM.
There are several reason of missed or delayed period. First exclude pregnancy
and you also get negative results several time.
Those are like hormonal
imbalance,PCOD,over wt,Psychological upset,Eating habit disorder
,stress, thyroid problem etc.
So need some investigations like USG lower abdomen,LH/FSH,PPBS,Thyroid profile,Blood estrogen and progesterone
Then consult with gynecologist with all reports and take advice accordingly.
Control body wt,avoid stress,take healthy diet and need proper sleep.