My menstrual cycle has changed, am menstruating for 21days sometimes or 5days after 3days i start again.what could be the cause and what do i take?

Common complaint: 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.