Might depersonalization/derealization be caused by allergies?

No: Depersonalization and derealization are usually form of emotional detachment to deal with a difficult situation. Allergies would cause some congestion and fogginess.

Nope -- different: Allergies can make you feel woozy-headed and less alert -- especially if taking sedating antihistamines, etc. But this isn't the same as depersonalization or derealization, which are more mind-altering experiences than this. These symptoms may come with various kinds of anxiety, substance intoxication or withdrawal, and some types of psychosis. They are involuntary also.