Welcome to the HCM
You had sex on 19th march and took post coital contraceptive
pills in the first 24 hours after having sex. It is the time when the post coital contraceptive pills are effective with a success rate of more than 95%. You had your cycles in start of April which means you were not pregnant upto that time. hopefully you didn't had sex again which means there should be no pregnancy
. but if you had sex after the 4th April then the date of conception is the same as that of having sex but if you have not had any sexual encounter after the 4th of April then it is more obvious that the test report was false positive.
I would suggest you to confirm the pregnancy by doing a blood test
levels" before going for any further work up.