My dr thinks im having breakthrough bleeding been bleeding 15 days. Can it get heavier?

Abnormal bleeding. In general, breakthrough bleeding is unpredictable. So, the short answer is yes. The bleeding could get heavier or start to improve. The real question is-were you examined? Did your doctor check for other causes of unpredictable bleeding such as fibroids, polyps, cervical dysplasia, etc. Make sure your questions are answered by your md to your satisfaction.