What does it mean? Am I depressed?

Over the course of the past few years, I have realized something odd about myself. Firstly, I'd like to say that I don't do drugs, so that isn't the problem, but I am a teenage girl, so this could simply be hormones? So, I found myself shying away from crowds of people. When I'm with my family at get-togethers, I have fun in the beginning, but later isolate myself. Randomly, small things can disrupt my tranquil mood and make me feel depressed. Sudden waves of sadness, grief, and despair come over me for no good reason. Then again, sudden waves of bubbling happiness burst out occasionally, too. Usually, the happiness lasts a matter of minutes, but the sadness lasts longer, perhaps hours. What does it mean? I didn't think hormones could do that to a teenager. The depression feels so intense that it makes me feel like I don't matter in the world and that I can't connect with other people. At the same time, I feel emotionless and empty. I'm certainly not suicidal or anything. It's just these weird moments of powerful emotion. Just wondering why this happens. If it is hormones (because I feel like everyone will say it is), how does it work?