It just doesn't want to leave me alone

Less than an hour ago I felt calm and contentment. I knew that I was feeling normal. But now... depression is creeping in. Slowly. Ever so swiftly. I have no idea what started it. I couldn't pin point on anything. Yet, this feeling is so strong and so affecting. I hate this feeling. It puts a scorn on my face. It droops my eyes. It shortens my temper. It just creates so much negativity in me. And I am hopeless against it. What I can do is to wait for the rage to pass :(