How do you know if you got a false positive? And why/how does it happen?

I got a positive yesterday using morning pee and it was positive within 3 seconds and it was a very clear line. However my period isn’t for another 2 days so I keep wondering how the fuck did I get a clear result that early and that quickly? So now we think it’s a false. I’m retesting on Saturday but idk.. I don’t wanna wait that long (I have to since I’m out of town).. by the way it was a first response test