Depends on your cycle. Implantation happens around 6-12 dpo. Your AF was 6 days, and you're 7 days after your period, which would leave you at cd 13. How long are your cycles? I take it you don't track your ovulation, so you'd have to calculate that in order to know how many dpos you are or might be. CD13 is very early. If we take an average 28 day cycle, and let's say you have a regular 13 day luteal phase, your ovulation would be around cd 14. Or other way round: let's say your implantation would be 6 days after ovulation, so very early, would mean your ovulation would have been right after your AF ended. So your normal cycle would be 20 days. It's not impossible, just very early?