False negative or chemical pregnancy?

"Chemical pregnancy" is a clinical term used for a very early miscarriage. During a chemical pregnancy, a pregnancy test starts off being positive... but soon after implantation, the embryo dies and it can't be seen on an ultrasound scan. It is hard to say what really happened in your case because you had the ultrasound scan too late after the positive pregnancy test on the 6th April. According to your last period on the 28th of February, a positive pregnancy test on the 6th of April would mean that you were 5 weeks and 2 days pregnant at the time. If you have taken the ultrasound earlier, you would have probably seen the gestation sac in the uterus and the pregnancy would be have been confirmed. The opposite (no gestation sac) would suggest a false positive pregnancy test-result or extrauterine pregnancy if the test was still positive. At the time of your ultrasound scan and another pregnancy test, 11 days later, the sac could have disappeared and hormonal levels (HCG) dropped. Now we can only guess what happened.
It seems that you had a false positive result because if you had had a chemical pregnancy, you would have had vaginal bleeding (period) during the miscarriage; chemical pregnancies end with vaginal bleeding. In the case of a so called "missed abortion", where a dead embryo is not expelled from the uterus, pregnancy tests are also negative and there is no vaginal bleeding... but an ultrasound scan will detect the dead embryo in the fetus. So, the only possibility left is false positive test.
Further, you might be experiencing a functional ovarian cyst which produces hormones that delay the menstrual period. It doesn't seem likely that you are pregnant right now. To get pregnant you should first wait until you experience another menstrual cycle as the present cycle has not yet finished (no period has come yet). Nonetheless, you can take a pregnancy test to assure yourself. And if you don't get your period soon, you can consult a gynecologist for more professional help.