You may be experiencing what is known as discontinuation syndrome. Discontinuation syndrome is a set of symptoms such as fatigue, nausea, muscle pain, insomnia, anxiety, agitation, dizziness, blurred vision, irritability, tingling sensations, vivid dreams, sweating or electric shock sensations, which many people experience when they abruptly stop taking an antidepressant which has serotonin re-uptake inhibition as part of its mechanism of action. Some people will also experience these symptoms when they miss a dose or take it late, especially if they are taking an antidepressant with a short half-life. Antidepressants with short half-lives are eliminated from the body so rapidly that symptoms may appear fairly soon after a missed dose, sometimes within hours. One example of an antidepressant with a short half-life is immediate release Effexor (venlafaxine), which has a half-life as short as three hours.
Source:
Warner, Christopher H. "Antidepressant Discontinuation Syndrome." American Family Physician 74.3 (2006): 449-56.
