Some call it "the blues," or "feeling down," or "temporary minor depressive tendencies."
There is good news, though, and it's backed up by scientific research, according to this quote from Flourish: A Visionary New Understanding of Happiness and Well-being, by Martin E. P. Seligman.
OTHER PEOPLE are the best antidote to the downs of life and the single most reliable up. ...My friend Stephen Post, professor of Medical Humanities at Stony Brook, tells a story about his mother. When he was a young boy, and his mother saw that he was in a bad mood, she would say, "Stephen, you are looking piqued. Why don't you go out and help someone?" Empirically, Ma Post's maxim has been put to rigorous test, and we scientists have found that doing a kindness produces the single most reliable momentary increase in well-being of any exercise we have tested.
An old song we used to sing in Sunday School comes to mind: "Jesus then Others then You; what a wonderful way to spell JOY."
Truth is...getting our minds off our selfish wants and focusing on helping others with their needs is bound to lighten the emotional burdens we bear.