What Can Be The Reason For Missed Periods?

I just had a quick question.. i got my last period the day after thanksgiving.it was the usual length, and the usual amount.But I didnt get in the whole month of december. But i have been getting my normal period systems..( my face breaks out, loss of hair, cramps, tender breasts) however, the month of December i was extremely stressed out to the point where I got a knot on the back of my right ear. Am I pregnant, or did I just miss my period due to stress?