On my last period I had a blood clot, and painful cramping, this month I am late 5 days and I know for a fact im not pregnant, what could this be?

Hormonal changes. Many women have irregular, dysfunctional, vaginal bleeding. If you are trying to become pregnant you should see your doctor.
Not unusual. It could be many things. It could be just late, or you could have a miscarriage. What ever is, it is quite common. Don't worry about it. Anytime if any fertilized egg is not compatible with life, it generally is aborted.