What Causes Stoppage In Menstruation After Childbirth?

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I stopped breastfeeding my 8 mo old 5 weeks ago. I started alternatining formula and breastfeeding two weeks before that. I have not had a period since I gave birth. Last Sat and Sun (about a week ago) I had very faint pink spotting only when I wiped and for the last two days (Wed and Thurs) I had brown spotting and some pink blood on a panty liner only after I went running. I have had no spotting today. I did not run today either. We had unprotected sex almost 3 weeks ago. At home pregnancy test was negative today. Could this be implantation bleeding or just a slow return to my period?