(3) Where, in an appeal under this Act, a penalty assessed by the Minister under this section or section 163.2 is in issue, the burden of establishing the facts justifying the assessment of the penalty is on the Minister.
The number of days in your cycle is calculated by counting the number of days from the beginning of one period to the beginning of the next period. If you have a short cycle, for example 21 days, and you bleed for 7 days, then you could ovulate right after your period.
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.