What causes a positive test one day and a negative the next?

So I’m 10dpo and yesterday morning I got a faint line on an Frer Test. Showed up in 3 mins and was pink- defo wasn’t an indent as I’ve had this before and this line was very different.

This morning I took another and didn’t really see a line... took a clear blue at the same time and that line was superrrrrrr faint. Showed within like 2 mins.

I was so sure it wasn’t an indent as it was so different to all the other indents I’ve ever had. But now I’m really confused by today’s response.... should I test again tomorrow ?? Does anyone know what can cause this? I don’t want to be unrealistic but I’m trying to be positive that I could still be pregnant!

I’m trying to be a bit hopeful instead of down about it all the time so no negative comments please I don’t need it!! But any help is appreciated x

Thanks 😊