Is pica only in non-food substances? for example, could it be a strange food that one suddenly seeks interest in?

Only non-food: Pica by definition is only a craving for non-food substances. As long as the desire for other foods is not causing you to neglect other parts of your diet, i would not worry.

Pica is nonfood: Pica is non-food substances. Cravings for food can have many physical causes- nutritional lack, endocrine issues, pregnancy, med side effects, etc. Psych issues such as stress, OCD, and eating disorders can cause this as well. If this is causing disruption or goes on more than a week, talk to your MD.