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.
Women who have regular periods can easily calculate when ovulation occurs. Ovulation almost always happens 14 days before you expect your period. For a woman who has 28 day cycles, that would be in the exact middle of the cycle, 14 days after the last period and 14 days before the next one. Women who have 30 day cycles can expect to ovulate 16 days after the last period, which is 14 days before the next one. Therefore, it is not possible to get pregnant on the day before you get your period, the last day of your cycle. There is an important exception to this, however. Women who have irregular cycles cannot predict when will get their next period.