Am I depressed? Help?

So, I feel strangely sad and I have started thinking I might be getting into depression. I feel like there's no meaning to life. I am bored to do anything. I feel stressed for no reason. I don't wanna talk. I just get agree when other people ask me if I'm okay. Sometimes I feel happy. I thank the God I have happy moments. But is there really any kind of depression that has breaks? I mean, it sounds silly. The most of the time I feel extremely sad, but there are some happy moments too. How can this happen? I think depression doesn't stop for an hour and then comes back. Anyway, I have lost a big amount of weight too. It didn't happen because I'm sad. I went on a diet cause I had some extra pounds. Do you think that loosing weight can make you sad too? My diet is healthy that's for sure. Lots of fruits and vegetables, proteins..e.t.c It's not like I have lack at something. I don't know, I'm confused. Is this depression?