If Pregnancy Test is Negative and is confirmed by Transvaginal Ultrasound then the following could be the possible causes of your Missed Period.
No 1 : Emotional Stress or illness or Travel
Anxiety can cause a delay in your periods for a few days and if you are under a lot of stress then it could be delayed even longer and so does travel or any illness.
Increased stress leads to rise in cortisol. Cortisol interferes with progesterone production and since cortisol is a stress hormone and in times of stress the body goes into flight or fight mode and normal menstrual cycle is disturbed, as the body thinks of it as an non-essential function in times of stress.
Working on the problems which are causing you stress could bring back the periods to normal.
