Had sex on dec 31st... started spotting jan 4th... it's now January 8th and I'm still spotting mostly red but sometimes brown... my period is due Thursday jan 12th... is it possible to get pregnant that fast ? Or what could this be ?

It could be, or it may not be. Just no way to tell at this time.
Not sure what you mean by pregnant that fast ? It only takes one time.
You will have to wait to see what happens when it is time for your period this month