What Causes A Delayed Menstrual Period?

I wanted to know if i could be pregnant or what could be going on with my body? Ok my periods are irregular i believe due to stress, but when I do come on it last for 5 days I have came on in october 26th and November this year but on November 19th I came on and it last for just that day and it was pinkish and I was cramping but this month (December) I haven't seen my period at all it usually come at the begining of the mouth. So could I be pregnant if so how many weeks or months?