I am thinking that it would be too early for you to be experiencing sore breasts if it was due to pregnancy. I will say that since my miscarriage I have the worst PMS ever. I get sore boobs, bad cramps, and even nauseousness....which I never experienced before.
I had read before that it was common to experience pregnancy symptoms before a period after you have had a miscarriage. Like how cruel is that..but it happens.
Implantation bleeding is usually pretty lite and doesn't last really long. It is also usually lite pink or brown in color.