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.
You'll start to notice signs that you're fertile about five days before you ovulate. 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.