Hello Friend, Good that you are seeking a help here. Yes, overthinking or worrying about past incidents causes depression. As body and mind are interlinked, such negative thoughts could influence physical body condition as well. One of the reason for her urticaria could be such depression.
First Consult a psychology counselor to allow her to openly discuss and expression her emotions, feeling and old experience. This will help to isolate the root cause and recommend a suitable solution.
Next Steps
Consult a psychologists directly or online.