Well, you had sex at a time that starts the more fertile period of your cycle, cycle day 10. (cycle day 1 is the first day of your period). Sperm can live up to a week. You probably ovulated around day 14 or 15 if you have an average cycle. So, it could be spotting from ovulating or yes, implantation spotting. Not too many people really get spotting when they have an embryo implanting but it could be. The timing seems to be right for all of this to happen. You can use an early detection pregnancy test-- I got a positive 4 days before my period was due with that type of test. Would this be good news?