If you're in a bad mood, especially when you were in a pretty good mood this morning, you can say you're moody. A moody person's emotions change unpredictably and often.
Someone with erratic moods is moody — you could also call them temperamental or changeable. If you describe a painting or a piece of music as moody, you probably mean that it has a dark, gloomy nature. A moody play, for example, might be a little depressing, or at least melancholy. Moody, from the Old English modig, began by meaning "arrogant or proud," and around the late sixteenth century became "subject to gloomy spells."