One possibility is a chemical pregnancy. I took a home pregnancy test and even the digital one which said "Pregnant". By the time I got in to see the OB - about 6 days later, I got a negative blood serum test. In a chemical pregnancy you are actually pregnant, but it fizzles out for lack of a better word.
Or
Maybe you got an evap line or read the test beyond the time specifications listed in the instructions.
i don't know, but it has happened to me before! was it a blue dye test, they are notorious for false positives.