First period in 5 months has now lasted 46 days. Started with old dark blood and clots only but is now bright red heavy blood. What's going on??

Irregular periods. 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.