Have you run a mile or 2 miles or more? Or it can happen if your taking birth control. Some birth control make you have 2, 3 or even 4 period a year. If you run 1 mile or more that's why your period off that long.
My Period Is 48 Days Late, But Im Not Pregnant. What Is Wrong?
If you are old enough it could be the menopause. If you had unprotected sex then I would be inclined to take another test to make sure.