False negative

I was told by my nurse to check to see if I'm pregnant on Sunday with an at home pregnancy test. I stated spotting on Saturday so I thought I wasn't pregnant. I checked anyways on Sunday and it came back negative. I went to the doctors office today for my next clomid round. … I had to pee in the cup for the doctors office. When my doctor comes into the room she tells me I'm pregnant. What!?! Hoping for my Valentine's Day baby!