Could be that you ovulated earlier then you thought. False negatives are much more common than false positives. Congratulations on your pregnancy! Sounds healthy to me.
The best thing you should do is call and make a doctors appointment
That happend to me with my first, the line was so light and hard to tell at first than in less than 2-mins it disappear! Later that week I took another pregnancy test and came out positive! So I check the first pregnancy test (just cause) and the line had came back and it was dark and so clear!!
I got a positive pregnancy test when I was 2 weeks pregnant.