UGH IM SO CONFUSED!

I'm confused and dont understand. Why do some test show an evap line and others dont? Even with the same brand. I took a test yesterday and got what I thought was a bfp because it was in the time frame and very dark for an evap although it was thin. Posted it and everyone told me its evap not bfp. Took another test today and its bfn. Nothing. Not even an evap and its the same brand. I'm supposed to get AF tomorrow but the only PMS I have is sore boobs. I'm nauseated, and have very little cramping. But I have a watery kinda creamy at times cm. I just dont understand.