I have questions about my son's paternity ...

Hello Good morning, I have doubts about the fatherhood of my son. In November I took a pill the next day and my last menstruation was 11/12/2011. I had unprotected sexual intercourse with a partner on 16,17,19 and 21 December . I had sexual intercourse with a second partner on the 26th at 11pm. almost on 27th, with masturbation (I do not know if the hand was with something but apparently not) but the penetration was protected! My ultrasound performed on February 25th says my fetus is 8 weeks and 5 days that is, the date of fertilization was December 26. with more or less 5 days of amrgem error. Could I have gotten pregnant on the 26th even using condoms? I'm desperate, help me