Why am i suddenly so depressed?!

You might be thinking something due to which you get depressed, it is better you try to sleep and take rest properly. Give rest to your mind. Dont think unusual things, think about the things that makes you happy.

Yeah, perhaps that is why. It may help to try and sleep, in the morning you should feel better. If not, then you could just be having one of these 'moments'. Happens to everyone.
It could also be because your energy has died down for tonight, or maybe it's too much energy that your body can't handle it.