I thought I was 8 weeks pregnant according to when my last period was (11-19-2013) was the first day of the last period and it ended on 11-25. I went for my first ultrasound today and all we seen was the sac. Is that normal. The doctor didn't show much emotion but she said I "should" be around 4 weeks how is that possible when my period was so long ago? (I do have irregular periods.)

This is something to discuss with your doctor. The presence of only a sack, doesn't promote your theory of being 8 weeks along.

You got pregnant perhaps 3 or 4 week after last period. Thus the fetus development 8 weeks after last period would be 4 or 5 week.

Particularly in your situation, going by your last menstrual cycle won't be enough for dating purposes. With it being irregular, you very well could have become pregnant after you had thought.

They will likely do another scan within a couple of weeks and should be better able to determine how things are progressing.