Suzan, like Wobbles said, you could have ovulated a few days earlier than you thought. Especially if you are not using opks or temping. Last month Fertility Friend predicted that I would ovulate around day 18. I used OPKs and temped and found out that I ovulated on day 16 (even though I got a positive opk on day 15 and 16). If I had not tested using an OPK, I would have assumed that I ovulated on day 18.
So, there is a chance that this is implantation spotting. How exciting!!! I really hope you are preggers Suz! Good luck. Don't get too stressed about it though.