Light spotting can be implantation or premenstrual. Since you're a day late, have you taken a test yet? Now would definitely be the time.
I had some highly unusual premenstrual spotting two cycles ago. I was convinced that I was pregnant, but then my period started after 7 days of nonstop spotting. It turns out that I had been so stressed out over getting a BFP that my emotions were affecting my cycle, which is a common thing with females who get heavily stressed for several days. But don't count yourself out yet; a new cycle does not begin until there's full flow, not just spotting.