Modafinil is a psychoactive drug and cause changes in perception including hallucinations; consult a doctor and discontinue use immediately if you experience hallucinations.
Hallucinations are caused by an altered functioning of the brain which may be caused by some substances, trauma damage, brain overstimulation (i.e. emotional states) or psychogenic (mental) disorders.
Absolutely. Type 1 Bipolar Affective Disorder (what one would classically call "manic-depression") at its most severe can cause psychotic symptoms of all sorts, particularly in the manic...