Welcome to HCM.
You have not mentioned anything about the duration of your cycles ( 28 day or 30 days).
I consider it to b a 28 day cycle. In that case you are more likely to be pregnant due to sex with your husband. Ovulation usually occurs 14 days before your periods start. As your LMP is 19 Dec
2011. Your next period is supposed to start on 16 jan. AS you have had sex with your husband on 31 dec and jan 01, they correspond to 12 & 13th day of your cycle. The viabilty of sperm is 36-48hrs and that of ovum is 18-36hrs.
Based on all the above things you are mostly likely to be pregnant because of your husband. Even if your cycle is of 30days, the chances of you being pregnant with your lover is very very rare.
Wish you a good Health.