Hello from JustAnswer.
I am sorry that you are having such problems.
This is most likely an early spontaneous miscarriage. It is possible to have a false pregnancy test result, but it would be extremely unusual to have three false results. So, it is far more likely that you were pregnant on the 18th and 19th of Sep when the tests were positive.
However, now with a negative pregnancy test and nothing to see on today's scan, you likely had spontaneous loss of the pregnancy, and it is likely that it occurred some time last week.
As the technology has improved to perform early pregnancy tests, it has also identified a large number of early miscarriages that years ago were never identified. At that time, these episodes were simply considered to be a late period. Current studies using these sensitive pregnancy tests show that early miscarriages occur in about 25% of pregnancies.
There is no reason to be concerned about having an early spontaneous miscarriage. It does not indicate any health issue in the mother. If you are intentionally trying to get pregnant, then you may continue to try to get pregnant as usual.
If I can provide any additional information, please let me know.