Blood Clots during Period. It is normal to pass blood clots during menses, as along with the blood the thickened lining of the uterus is also being shed and expelled outside the body. In case the abnormal blood clots during period occur, visit your gynecologist as it may then be an indication of other serious medical illnesses.
And yes, it's still possible that you're pregnant. In a normal 28-day cycle, your period comes about 14-16 days after ovulation. However, unless you are positive that you know exactly when you ovulated, your result could be off one or two days. If you did ovulate on the 20th of April, that is 15 days ago. If you're not pregnant, I would expect you to start your period in the next day or two.
Ovulation usually occurs between 12 and 14 days before your period starts (Fehring et al 2006). This is an average, so it could be a couple of days earlier or later. For example, say you have a regular 28-day menstrual cycle. Count the first day of your period as day one. Your fertile window is likely to be around days 10 to 15. However, a lot of women have an irregular cycle. If your cycle is irregular, ovulation may occur a week earlier or later from one month to the next (Fehring et al 2006).