Hmmmm I felt very down today?

For absolutely no reason at all, I found myself feeling weighted down. But i had nothing to be sad about. I mean, it's Friday, and I should have felt happy. The day went by easy enough. But I found myself feeling down. When my friends talked, I couldn't find the enthusiasm that I usually have to continue carrying on the conversation. They kept asking me why I was so quiet and I didn't really know why and it bothered me. everything is going normal at home, i really have nothing to be sad about. but i just felt blah. Then my BFF said: "Are you ok? Is there anything we can do?" And I felt really bad and I told her I was fine just tired. So during lunch I forced myself to smile and laugh with them but I really wasn't feeling it... anyone feel this way before? I'm 16. Thanks.