What Causes Delayed Menstruation?

My period is almost 6 days late now, but I know I'm not pregnant so that's not a concern. I've been having cramps on and off the last few days and now I have a fever but I'm not sick.. The only symptom I have is a bad stomach ache. The only thing that I think could be causing the late period is that I have exams this week and I guess that would cause stress to make me have a late period but I don't feel stressed at all. I also noticed I lost about 3 lbs in the last few days, but I don't know if thay has to do anything with it. So, I was wondering if you knew the cause of the late period.

Thanks