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.
There is not a way to know for certain how long after your period you will ovulate, however. Ovulation does not occur in relation to the previous period, but rather to the next period. You will ovulate 14 days prior to the start of your next menstrual cycle. This means, then, that if you have a 28-day cycle, you will ovulate roughly 14 days after your period, and 14 days prior to the start of your next period. If you have a 31 day cycle, however, you will ovulate roughly 17 days after your period, and, again, 14 days prior to the start of your next period. If you are trying to conceive, you should begin to have intercourse roughly 4-5 days prior to ovulation, as the sperm can live in the body for about 5 days.