Faint negative pregnancy test result?

I was on birth control for roughly two years and stopped taking the pill about 3 months ago. My husband and I have been having unprotected sex since then. I am currently 9 days late on my period, although I had some very light brown discharge for a few days when I should have had my period. I took a pregnancy test yesterday, when I was 8 days late, and I got a negative result almost instantly but it was very light. I plan on retesting early next week if my period hasn't came yet. Is there a reason why I got a faint negative rather than a definite negative?