Welcome to HCM,
As per you description you had sex on last day of your period that is 17 march on last day of period there are chances of pregnancy
are almost nil, as it is safe period and ovulation takes place approximately in mid cycle 14 days before expected date.
You took nordette
which is used as emergency contraceptive pill
and claims to be effective up to 120 hours of unprotected sex.
Your urine pregnancy tests are negative and blood Beta HCG
is also negative so please do not worry you are not pregnant.blood HCG is accurate.
Due to those hormones period can get delayed and irregular, if you have any doubt then visit Gynecologist and get Ultrasonography
Please practice safe sex as it protects from unwanted pregnancy and STDs.