What Causes Delayed Period While UPT Is Negative?

Hi, I am a 29 yr old and my weight is 52lbs and my heigt 5.5'. I used to have delayed periods since I had my first menstrual cycle usually 35-40, but regular cycles every month. After my marriage in the last 4 months, it became very regular exactly 30 days. And am not on any contraception now. But this month, it is delayed by more than a week, although I started experiencing my PMS symptoms. I too a home pregnancy test on the day I was supposed to get a period (30th and 31st day) and it came negative. However, I did not test again. Also, I was more stressed this month because of my work but not before a week from my expected menses date. Is it possible that my ovulation may have started late?