Confused ?

I tested Saturday the 20th, but it was negative. I looked on here to see when AF was due and it said I was supposed to get it from the 20th-23rd of this month. I'm very confused as to why it would have been a negative, but I've missed my period? It's never been late. And I've been cramping in my lower abdomen non stop and it feels like period cramps but waaaaay worse. I talked to my friend who is pregnant and she said she had these cramps also when she was first pregnant. Can anybody tell me what's going on?