Up and Down....

This few days my mood was up and down. All of the sudden i'll be very happy den after that i'll be feeling sad....What happen to me? Can any of you tell me?Even myself also don't know what am i thinking? Is that the matter which affects my emotion?Or what? Although i'm having lots of faith towards God and myself but sometimes i do feel very miserable. I try to search my personality back but i can't get back all of them...Is that everyone's personality will change when time passes by?Or just like what i told dummy that:" Everyday our earth is turning around and around, and people who we knew are also changing day by day"......

What can i do?