My daughter had this exact same thing when she was 2...her feet/hands swelled up so badly that her nursery school called us & we ended up at the childrens hospital. They couldn't tell what caused it but prescribed an epipen just in case, sent her for allergy testing which turned up nothing. For a month after the initial swelling, she would randomly get hives in different spots which would go away with benedryl. The allergist said it was probably just viral & not to worry & we've never had it happen since.