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.
Time of ovulation. Women often believe that ovulation occurs mid-cycle. It actually occurs 12-16 days before the next period starts. So, although a woman with a 28-day cycle may ovulate mid-cycle (between day 12 and day 16), a woman with a 36-day cycle will ovulate between day 20 and day 24. For women with regular cycles, an easy way to approximate the time of ovulation is to subtract 16 from the number of days in the cycle and then add 4.