Strange mood

It's strange how a person's mood could change from one moment to another. This morning I woke up all tired but yet happy. I walked out in the rain and took risk course 1. I walked all the way back home and started helping mom at home. Cleaning and stuff. All of the sudden I felt so lonely, so sad and I had to fight my tears back. For some reason I'm down and I haven't been able to give my family a proper smile. I do feel better and I tell people I'm fine, but when stuff like this happens it makes me reconsider the matter.
Am I such a good liar that I have even got myself fooled by my own lies? Life is short and I know I shouldn't give waste my time on being sad and just live my life. It's much easier said than done.You could ignore the fact that you're not happy with something and just go out, have fun and party but at the end of the day when you're all alone those feelings get back to you.
Not knowing the reason of your "depression" (doesn't have to be the kind of  DEPRESSION you learn about in psychology) makes it harder for you to work it through and get over whatever it is that's weighting down on you.
So what's the solution? Is there even one? I don't know but I will surely look for one, because this is not okey at all!