When we are not happy - unhappy - we usually use the same constantly word: I am sad... So, English isn't a boring language that you can use only one alternative to express your feeling so, I listed few options below:
to be feeling blue:
This morning I talked to my cousin... She is feeling blue because her cat has died.
to feel down;
Here where I live is always raining... I feel down with this horrorible weather! I wish I could live at the beach.
to be down in the dumps:
Every time he watches this movie he gets down in the dumps because it reminds his girlfriend. He is such a romantic guy!
to be feeling gloomy:
Today I am not okay,,, I don't know why, but you know these day when you are feeling gloomy and nothing can cheer you up?! That's me today... I just need my bed and an ice-cream bowl.
Do you know any other expression? Write down on the comments. Let's share! And remeber: don't worry cause every little thing is gonna be alright!