23 year old girl presents with 86 day amenorrhoea with previous 40 day regular cycles. She took primolut 5mg once daily for 5 days on 76th day of amenorrhoea. There is no withdrawal bleeding yet.No hirsutism, no acne present.No history of sudden gain in weight.

Thyroid profile is normal. Patient had similar episode of amenorrhoea 5 months back and resumed menstruation after 71 days without taking primolut. What can be the most likely cause and investigations need to be done?