What Causes Light Spotting With Brown Discharge?

Hi, i had a baby march 25th and just a few days ago starting what i thought was my period, its was bright red i seen on the first tampon but after that it has been dark brown , kind of muscusy and light in terms of volumn . i had unprotected sex the same day just before i noticed the bleeding, do you think this bleeding was brought on by sex? or is this suppose to be an actual period. Help me! Thanks