I can be happy but then get mad or sad real easily. Whats wrong with me?

Maybe nothing. Everyone (especially teens) have mood swings. If yours aren't too extreme and don't interfere with your life, it may be normal. On the other hand, some people are seriously angry or sad underneath, but hide it until "triggered" by something. There are also some medical conditions that could cause this. If it's a problem, see your doctor or a mental health professional.
Talk to therapist. Your mood swings may reflect a variety of things including anxiety, negative self-esteem, and/or depression. Insight or psychodynamic therapy can help you discover what the underlying issues are that make you so emotionally labile. Good to address at such a young age so that this does not become a life long response pattern.