implantation bleeding?

I did a 50km walk on Saturday, and had light spotting just once that morning. Yesterday I experienced a little spotting in the day then in the evening it was a bit thicker, like a pinky colour when I wiped (TMI!) 
I've had it on and off again today, definately not my period as not due on for another 10 days. Could it be implantation bleeding? Would appreciate the help! 

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