What Causes Short Menstrual Period And Stomach Pain?

Hi. I'm 18 and although my periods usually last around a week, this month I only bled for 2 or 3 days and then just stopped. I found a few spots of dark blood the next night and I am still having stomach pains (I normally get period pains). I just thought it was really odd so wondered if it's just nothing or if I've done something to cause it? Thankyou.