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.
A period starts two weeks after ovulation if no pregnancy occurs. (It can be 13 or 15 days, but 14 is pretty accurate, so let's just go with that.) If a woman has a nice regular 28 day cycle and a 4 day period, that means that she will ovulate 14 days after her last period started and 14 days before her next one is due. That also means there will be at least 9, probably 10, days after her last period ends before she ovulates.