What Causes Delayed Periods While UPT Is Negative?

Hello doctors! I just want to ask about my situation. I usually my menstration is regular monthly. But now My period was delayed since december 23. Negative pregnancy test, negative hcg, negative for transvaginal ultrasound. Hormones are normal thru blood test. My doctor prescribed me duphaston 10mg three times a day for five days. My questions is, what are the possible reasons why my period is delayed? When i will be expecting my period to come. And what will happen if my period did not come after 7days of taking the duphaston. Thank you in advance