My period ended a week ago now im having severe back cramps and heavy bright red bleeding. ive had my tubes tied. what could it be?

Irregular period: Different bleeding patterns are not all that unusual but to be sure make sure you are not pregnant as this could still be a possibility. There would be a high likelihood of an ectopic pregnancy. If the pain is bad and the bleeding is heavy you may want to see a doctor. If you are in a bad way then consider the ER or decide if you can wait, depends on how bad you feel.