Hi Good Day,
Welcome to HCM.
If your periods are regular, you should already have had your next period by 28th of April. Since that did not happen, your ovulation could have been delayed, in which case you could be possibly pregnant. There is no way of being sure that you are not pregnant until you have your next period.
Meanwhile you could get your serum / urine pregnancy
test done to confirm pregnancy, though Negative result does not exclude pregnancy, it only means that pregnancy is not of minimum duration which is required for the test to become positive. Telling in percentage will be pure speculation.