Period or implantation blood??

I had unprotected sex and he came inside me last Tuesday- I was supposed to start my period a few days ago and my boobs were hurting so I guessed I would start soon. Yesterday I felt fluid and when I checked it was dark brown blood, not like my normal period. It has lightened up both in flow and color...I am usually regularly heavy in the beginning of my cycle. I tested 2 days ago out of curiosity and it was negative but I read that dark brown bleeding could be implantation blood...my boobs still hurt. Any thoughts? (I will test again in a few days)