There are several reason of delayed period or OLIGOMENORRHOEA. You also exclude pregnancy
by negative test.
You are probably suffering from Ovarian cyst
or PCOD and other causes are thyroid problem, over wt or extreme wt loss, psychological prob, stress, eating disorder
I suggest you to do a USG for lower abdomen and some hormone profile like LH/FSH & Thyroid profile to know the underlying pathology.
Then consult with your gynecologist and take advice accordingly.
Be well and take care.