My period ended 6 days ago. Light and lasted 7 days. I woke up with bright red blood in the toilet. I needed to put in a tampon. What could be wrong?

Left over blood. Irregular periods are one of the most common problems obgyns treat. If this is one time problem it usually resolves on its own. If this is a recurring problem you should be evaluated. Infections ; hormonal changes can cause spotting. If there is a concern for pregnancy then do a home pregnancy test . There are many options available to help regulate the menstrual cycle. Schedule a visit.