I answered you but somehow it didn't post.
It's unlikely you got pregnant in October, since you had what appear to be two periods after that encounter.
Once you have your first doctor appointment, it will be clear though which man is the father - you won't be left confused about whether you got pregnant in October or in December - there's too much difference in the fetus between those two dates.
Once you have a doctor appointment, you can establish how far along you are through sonogram. Yes, 2 months difference is enough where you won't to be confused about paternity - it will be obvious if you are pregnant from October vs. from a December encounter.
I agree that you hear women say they had a period the first month of pregnancy - this used to be absolutely unheard of. I think it used to happen, but without the internet, you just never heard about it. The gold standard was, if you got your period, you weren't pregnant.
It's still very unlikely that you'll have what appears to be two regular normal periods and be pregnant. So it's very likely the encounter in December led to your pregnacy.