I think if you are questioning your emotions and feelings, then it might be helpful to seek a doctor's advice/diagnosis.

I don't know much about SAD, but I do know a great deal about depression. It's difficult to say how it affects everyone, but major depression doesn't come and go in a matter of a few hours. Major depression, by definition, is something that you literally have a difficult time functioning and pulling yourself out of. It could last from weeks to months, to years.

Given your age, could it be likely that you might be having some hormonal changes that are influencing how you feel?

I wouldn't jump ahead and think about medication this early in the game. Just consider going to talk to someone - maybe your primary care physician or your pediatrician (if you still have one) can help, especially if s/he has known you since you were younger. They might be able to work from a baseline since they have known you for quite some time. Again, don't put the cart before the horse, so-to-speak. Give it time and just talk to someone first.

Good luck.