I don't understand?

Ok so I'm 6 days late for af which means 20dpo. I am extremely moody. I am nauseous most of the time. My bbs are sensitive. I spotted brown discharge about a week ago for 3 days and it stopped. I am gassy and have weird cramps that go from my ribs and move everywhere below that. But I had bfn. I did have one test that was neg and turned positive overnight. My mom had the same thing with my brother. I'm just really confused. Why am I getting bfns? And if I'm not pregnant what in the he'll is wrong with me?