hello,
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,
hormonal imbalance etc.
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.