What are your cycle lengths like?
Being only CD13 I would be more inclined to say a breakthrough bleed (I think that's waht it is called - sometime when the egg is released it can cause spotting, not sure why though) rather than implantation.
BUT if you have shorter cycles and have been O'ing around CD9-10 then it could definitely be implantation.
Do you know if you have O'd yet?

Alternatively, if you have DTD recently it could just be a little bit of olf blood from the cervix, it's not uncommon to pop capillaries every now and then.

I am 1DPO and really hard as it is our last chance for #2 before we put it off.
OMG is that Harrison standing up?!! Crikey me!!!
Lots of Babydust to us both, would be wonderful if we could be bellybuddies together with our sons being the same age too