What Caused The Spotting After The Period Was Over?

My wife finished her period about 9-14 days ago and for the last 2 days she has had darker bloody spotting. We had sex the Friday-Sunday, about 2 days after the period stoped, after her period. We were apart for a week for work and then were sexually active the following weekend, Fri-Sun. Monday evening she starting having this spotting and it has continued thru Tuesday evening. She is not on Birth Control and we are trying to concieve. Should we be concerned? Could this be nothing or maybe Implantation bleeding?