I started taking allegra (fexofenadine) d for my allergies my appetite decreased. Is this one of its side effects?

Could be related. Allegra (fexofenadine) d has not been usually associated with decreased appetite. The Pseudoephedrine which is the d part of the Allegra (fexofenadine) d can cause nausea in very few patients and if that is the symptom you are experiencing then you wont experience this with plain allegra (fexofenadine).