I've learned that there's a difference between happiness and joyfulness. Happiness is something you create, something that you feel based on external (and sometimes internal) stimuli. Joyfulness is something that's given to you. You may not be happy or you may be down about something, but God gives you joyfulness anyway. I think it's best described by feeling happy and not knowing why.
Did anything special happen to you today? Did any outside factors contribute to making you happy (material possessions, people, nature, etc)? If none of these things were present, and you're still happy, then I think that's joyfulness my friend.
And I'm feeling joyful this week.