Spotting at 13 weeks

Hi ladies, i am 13 weeks pregnant with our 2nd child and me and the other half had sex 2 days ago and i have since been spotting dark brown discharge, its only when i wipe. I have no pain and had our 12 week 3 day scan 4 days agoand everything  was as it should be. This has happened before when we have had sex on 3 seperate occasions. Any ideas what is going on? Or had this before? Thanks