How can I tell if I'm having implantation bleeding or a light period?

I normally have very predictable periods, but this month has been different. I am about 4 days late starting my regular flow, but have been having very light bleeding off and on for the past 3 days. It started out very faint pink, and is now a dark rusty color. No clotting on anything like I usually have. I took a pregnancy test the 2nd day of my "missed" period before the bleeding really started and it came back negative. Could it have been a false reading? Should I retake a test in a few days?