It is possible to get a strong reaction to only a small amount of food. Children with severe allergies have been known to go into anaphylaxis after eating or even touching a small amount of their allergen.

Strawberries are a known allergen, so it could have been that. Do you think she is itchy? If she is it is more likely to be a reaction as opposed to a viral rash as that wouldn't worry her as much. Or has she been ill in the past few days - fever etc?