How to know if it's a period or implantation bleeding?

Gracie
So about a week ago I had unprotected sex, however my boyfriend didn't ejaculate.  I got my 'period' yesterday (one day late, nothing unusual) but it started flowing very slowly and with very dark brown blood. It is still quite light flow however the blood is now red. ( it's Usually very heavy) 
What could be causing the changes? 
Could this be implantation bleeding? Could I still be pregnant?
Thanks in advance xx

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