Very sorry to hear about the mixed results. It can be heartbreaking to have a positive result followed by a negative result.
One possibility is a 'chemical pregnancy'. This is a pregnancy where the sperm and egg meet but do not embed properly, usually resulting in a period which is later than usual. Unfortunately, it does seem to kick off the hormone process, which results in 'feeling' pregnant and may cause some physical symptoms of pregnancy.
Companies which make pregnancy tests like people to test as early as possible since that's clearly more profitable for them. But chemical pregnancies will test positive on a home test around the time your period is due, but negative on blood tests (or show that hormone levels are too low for a successful pregnancy). So the very fact that home tests are improved and can be used earlier can actually cause unnecessary grief for people who test early. These days, I always wait until I'm at least a week overdue before testing with a home test (longest week of my life!!).
There are other possibilities such as hormone fluctuations (if you have recently come off the pill, for example), and on very rare occasions there are problems with the blood tests. So keep an eye on things, and if your symptoms continue and you still don't get your period, I'd recommend heading back to your GP.
In the meantime, we're here for you.
All the best