Can my toddler's fussiness mean she has a food allergy or not?

Possible. Fussiness is not a very specific symptom & is associated with many complaints. When fussiness is related to something in the diet it's more likely an intolerance than an allergy. Your pediatrician or allergist can be of great help here.
Fussiness. Symptoms of "food allergies" include: rash/hives, nausea, stomach pain, diarrhea, shortness of breath, wheezing, anaphylaxis. "food intolerance" is a digestive system response rather than an "immune system" response. Common food allergies include: nuts, fish, shellfish, milk, eggs, soy & wheat. An elimination diet may allow you to rule out this cause for her fussiness. (i think it is behavioral!).