Thats good that they've booked you in for a scan! Yes it may be too early, but at the same time, it's good to check if there is any retained tissue, and rule it out! If they can at least rule that out, but still can't see much, they'll book a follow up scan so they can check the progress.
I don't want to be insensitive in any way, I just remember a friend having retained tissue after her loss some years back. But I'm guessing if there is any retained tissue, then it may make conceiving difficult until it's passed? I remember her saying she had to wait until they had made sure there wasn't anything left before they could try again. So if it is that, then at the very least, once any remaining tissue has passed, it should be all clear to try again. Then you'll be confident in the test result.
As I mentioned before, logically, it doesn't make sense for your previous tests a few weeks back to come back absolutely BFN and then to get some progressive lines now? Surely hcg shouldn't rise again, especially after a few weeks. So I have all of my fingers crossed for you!