That same exact thing has happened to me. When I went for my yearly checkup I asked my doctor about it and she told me that it was nothing to worry about. She said it was just a little 'old' blood from your period. It probally had nothing to do with sex. In fact, she told me that some women will occasionally do this mid month, well after their periods. So, I wouldnt stress out about it unless it continues. and if for any reason you are still worried, go ahead and make an appt to see a doctor. no harm in that!
"Brown: May happen right after periods, and is just "cleaning out" your vagina. Old blood looks brown.

Spotting Blood/Brown Discharge: This may occur when you are ovulating/mid-cycle. Sometimes early in pregnancy you may have spotting or a brownish discharge at the time your period would normally come. If you have spotting at the time of your normal period rather than your usual amount of flow, and you have had sex without using birth control, you should check a pregnancy test."

(I FOUND THE ABOVE ON A MEDICAL WEBSITE) GOOD LUCK TO YOU!