My girlfriend was supposed to stop her period today but instead has heavy bleeding what's going on?

Changes in bleeding: This can occur for many reasons from hormonal imbalances, stress, infection, pregnancy, bleeding disorders, medical disorders, growths..... If it becomes a pattern over 3 months, then follow up with gynecologist. If associated with heavy bleeding more than a maxipad full an hour, fever, odor, chills sweats, shortness of breath, pain then follow up ASAP or ER