if your last AF was on December 19th, it is a little too early for implantation bleeding. that usually happens 2 weeks to 3 weeks after AF is supposed to come.
I had implantation bleeding at 11, 12, and 14 dpo. It was just enough to wear a panty liner(sorry if tmi) and brown, nothing at all like my period. Implantation bleeding usually occurs when implantation happens, 7-12 days past ovulation. Hope that helps.
I think it could be... I've been researching I.B. because I was afraid that was the case I am in (after 12 days of clotted brown-black beeding I have basically ruled it out as a side-effect of my seasonale birth control). Most things I've read have said 6-12 days after fertilization (so anytime you've had sex in days 9-15 of your cycle would probably be most likely as a starting point for counting. I also read that 20-30% of women, especially first time pregnancies, experience this. So it's not like its unlikely at all. Other factors to consider... are you on bcp? do you have sex a lot? Sometimes a tiny bit of bleeding can just be due to irritation from penetration. I hope everytime works out for you. My advice would be to take a test after a few days if your AF doesn't come this month. =)
Best- Lauren