Implantation bleed?

Kathryn • Hallo :)
Hello! 
Forgive me if this is stupid but could I have had implantation bleeding today? We DTD on 5th January in my fertile window (not TTC but not preventing either!) and my period is due on Friday 15th (in 4 days time) 
I've had a little bit of spotting (brownish blood- sorry tmi) but nothing else. Could it be implantation? Could I be pregnant? 
Thanks in advance! 

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