Hi, beauly, what was the baby's estimated due date the first time you were given a due date? Was it from an ultrasound? If you had an ultrasound pretty early in the pregnancy, it might help throw some light on why the doctor said what he did.
Did you have sex with the first guy (the one you mention as your man) throughout that time, including after the sex with the other guy? Or was it just December 11 and that was it with him in December? I would not necessarily think you got pregnant as early as December 11, but he wouldn't be ruled out if you had sex with him later, like December 23 or at Christmas time.