This sort of thing happens to everyone
As human beings, our bodies aren't 100% perfect, sometimes we 'loose' the balance in our head slightly and that makes us feel depressed, or it could just be anxiety
There is nothing wrong with this, I don't know one person who doesn't feel like this from time to time, NT/AS all get this
If it is happening too often, maybe there is something bothering you, or you just need to give yourself a break from things
Don't worry about cheering up, because you will feel back to normal eventually, it will pass