On Dec 6-9th of this month I had my normal period had sex right before I got my period and since my last period I have been having sex with out birth control or a condom now I am having another period on Dec 27 and it is really dark blood but still kind of red. Is this implantation bleeding or am I having a second period in the same month?

I am always on time for my periods and everything never have I been late or to early nor had two periods in the same month.