the best and most accurate method for determining conception is your
cycle. If you cycled and have not been with guy B since that, it
is most likely that you conceived after your cycle. Remember that
your fertile days are limited to a few days each month. I suggest
looking up information on fertility and conception, then cross
reference your own dates and activities to bring yourself some peace of
mind. If it is really confusing you, talk to your doctor about
your time frames and see if they can offer any wisdom on it all.
From my experience with calculating fertility, it sounds like you DID get prego in January, not in december.