I'm a little confused ... who is "he"?? I'm guessing you've been away for a couple of days, so you might need to question "he" more closely and see if something happened or she was given something.
To be honest, anybody other than you is probably not going to be as careful purely because they haven't had to battle the consequences. So something very innocent that they thought nothing of, could have been given to her, or she could have come into contact with it.
Now she is in your care .. see if the hives start to settle down after a couple of days.