Hello
Thanks for writing to DoctorSpring.com with your health concern.

With three negative pregnancy tests, the likelihood of your being pregnant is almost nil.

So there has to be some other factor responsible for the delayed cycle (s).
Stress is ruled out.
So the other factors are - unaccustomed exercise, recent weight gain or weight loss, ingestion of certain medications that affect the menstrual cycle, late nights, erratic eating habits, change in climate or altitude.
Hypothyroidism and PCOD ( polycystic ovarian disease ) are two conditions that can cause these symptoms.

A normal ultrasound in August though rules out PCOD , however sometimes hormonal tests are required to confirm the diagnosis.

Please get these blood tests done - FSH, LH, TSH, Prolactin, OGTT, DHEAS, free testosterone.

Also, I hope the pelvic ultrasound scan was done transvaginally.
If not please repeat the scan transvaginally.

All the best.
Please feel free to discuss further.