Why!?

I just tested positive this morning. It'll be my first baby. AF was due 4 days ago. A few days before AF's due date, I started feeling something like light cramps, and I was certain she was gonna show her ugly face. Well I'm getting those light cramps today too even though I tested positive. Why is that happening? It's making me think my positive was a fake and AF is gonna show any second. Also, I had some light brown spotting last night which also lead me to believe that AF was gonna come. Can any one explain this to me?