I think I could be depressed, what to do?

Hi, lately I'm feeling really depressed and don't know what to do. I'm a top student and my grades haven't been dropping, but I'm continually gaining weight. I found out yesterday I gained 8 pounds in about a week. I've noticed that it's getting harder and harder for me to focus or pay attention to what I'm doing, and it's hard for me to find enjoyment in anything anymore. I tend to avoid being around people, including my friends. School's just ended, but in it I've never dropped below a 4.0 GPA, but I just don't feel competent at anything I do anymore. I used to aspire to get into high-end performing arts schools, but I don't think I'm at all good enough anymore. People tell me I'm smart, and a model student, but I just feel stupid and disliked lately. I have a lot of trouble sleeping, it takes me forever to fall asleep and I wake up a lot during the night. An hour ago I started crying for no real reason at all, and I have been having mood swings lately, but they're usually angry ones directed at my mom. Lately, I just feel like nobody will ever accept me and that I will never be able to succeed at any specific things. If it makes a difference, my mom and sister have both been diagnosed with depression. Anyway, after all that spew, back to my main question(s). Am I depressed? And if so, who should I talk to? I feel like if I talked to anyone in my family about it, they'd just see it as a cry for attention, which is the last thing I want.