Is my 16 year old daughter suffering with depression?

In the last few months I've noticed a real change in her. She's withdrawn from me a lot, she snaps a lot, avoids meal times where there is a lot of conversation, she doesnt come downstairs in the evenings anymore, she just stays in her bedroom. If one of her friends calls the house phone she looks at the caller id and yells for no one to pick it up because she says she doesnt feel like talking to them. She goes out a lot less and just sits on her laptop all day. (She's just left school and finished exams) When she does go out its often little walks on her own. She seems generally down and very easily irritated. It's just very different from what she is normally like. Does it sound to you like depression or am I simply being a neurotic mother?