Period or random bleeding? TMI

Stephanie • 12 mo daughter 💕 Due in August baby#2
I'm 18 days late on my period... My husband and I BD'd laatnight, and when I put a tampon in this am I had light pink bleeding not a lot... Now it's dark brown.. is this my period coming on? Or could it just be from us having sex last night ? I'm still having BFN.