I looked it up on Google health and found this:

Cyclothymic disorder is a mild form of bipolar disorder in which a person has mood swings from mild or moderate depression to euphoria and excitement, but stays connected to reality. The cause of cyclothymic disorder is unknown. Although the changes in mood are irregular and quick, the mood swings are far less severe than in bipolar disorder (manic depressive illness). Unlike in bipolar disorder, periods of hypomania often do not become actual mania.

Symptoms:

• Alternating episodes of hypomania and mild depression lasting for at least 2 years
• Patients tend to be more irritable or dark, rather than euphoric or happy
• Persistent symptoms (less than 2 symptom-free months in a row)

From Wiki: Diagnosis requires a history of hypomanic episodes with periods of depression that are not major depressive episodes. The diagnosis is excluded if there are manic episodes or mixed episodes, or if a number of other conditions such as schizophrenia or drug abuse are present

Here's a link to WebMD: http://www.webmd.com/bipolar-disorde...hymic-disorder