If you had a period from the 31st of October to November 6, that rules out sex before October 31 as causing the pregnancy. (I'm assuming you know that a period means you are not pregnant.) You said your "first positive pregnancy test" was November 29 -- from your wording it sounds like you had a negative test (0r tests) before November 29? If the sex from October 18 had produced a pregnancy, a test would have been positive by the 1st of November. (Except that your period came. Which meant, you weren't pregnant.) After your period came, if I'm understanding this correctly, you're saying you only had sex with the second guy. I'm trying to figure out what part of this story is worrying you.
You say "First ultrasound after Christmas -8 weeks." How many days after Christmas was that ultrasound?