Thanks for writing to DoctorSpring.com with your health concern.
Some common causes of a missed period - anxiety, stress, personal or professional tensions, weight gain or loss, late nights, erratic eating habits, travelling, change in climate or season, smoking, drinking , ingestion of some medications etc.
If these are ruled out, then indeed hypothyroidism, PCOS and endometrial hyperplasia should be ruled out.
For this you need - a thorough internal evaluation, per vaginum and per speculum check up, pelvic ultrasound done transvaginally and hormonal levels - FSH, LH, TSH, Prolactin, OGTT, DHEAS, Free testosterone.
As it was done in October, get a repeat anyway.
Also, the symptoms closely mimic endometriosis, which is often a clinical diagnosis (digital exam by a gynec ) and missed on even ultrasound.
Laparoscopy is confirmatory.
Please have an evaluation ( did you take a pregnancy test this time around too ) - also, taking oral contraceptive pills or cyclical progesterone for 3 months could jump start your cycle and alleviate most of the symptoms.
All the best
Please feel free to discuss further.