What's going on? Am I depressed?

How can you tell if you are psychologically depressed? How do you ask? What if you are only depressed sometimes? I am a first-year psychology student, I live in college, and I seem to be having plenty of mood swings lately. I used to be very bubbly and enthusiastic and happy and relaxed all the time, but lately, I've just been edgy and sad, and I cry a lot. What's going on? I've been at university now for nine months. I'm only seventeen, and I live away from home (and I'm happy about that). I still have fun sometimes, and I get involved a lot, but then something will happen and I just bomb. I feel helpless and useless and angry because I am unappreciated. It annoys my friends and that makes me feel even worse. What's going on?