@December23 when they say that you are 6 weeks, you actually did the deed to make the baby 2 weeks after this (so when you had the scan and was told 6 weeks, you conceived baby 4 weeks before the scan)
It's confusing, but they date a pregnancy from approx date of your last period rather than the date the egg was fertilised, so technically, you are "pregnant" 2 weeks before ovulation.
I hope that this makes sense! I think it's your new boyfriends baby, not the ex