Hi Stephie no great answers for you, but wanted to at least give you a bit of support as it is really hard not knowing.
This could either be because it is very early on yet so not enough HCG to give consistent results or because it is a PG that will not progress ( a chemical PG). If it is too early in a couple of days the results should be better, although i admit it is a loooonnnnnng couple of days.
I am in exactly the same position. Got a faint positive on thursday last week on an ic, bought a first response on friday and got a pregant on that, then used two more cheapies on saturday and got no second line.
Because of my history i'm supposed to have blood tests as soon as i get a positive so had one done yesterday, and will have a follow up tomorrow.That will tell me if the HCG is rising or not. My result from yesterday was very low but that could still mean because its either very early or because its ending. So i have to wait for the result to come back.
If you wanted clarity, is there a self referring early pregnancy unit near you that you could go to and ask for the HCG blood tests?
Alternatively you could wait a few days and try another First Response?
Sorry not to have a definitive answer, i know the waiting is dreadful. Good luck