My last period was November 22, I was with a man before November 22 and then I moved. I met a new man and had sex with him December 26. The doctor told me I was 6 weeks pregnant. Whose baby would it be?

No way to know without a DNA test after the baby is born. You'll have to wait to find out who the father is

Did you have a normal period (monthly cycle) between the two men ?

How did doctor determine 6 week.
More information is needed to determine. With 4 weeks separation, it may be possible, to tell better. Was a sonogram done ?

But I do agree, a DNA test at birth would be best to be sure