Thanks for the query.
she might have developed an allergic reaction
to any material while running in field.
once a person gets allergic, the body reacts to many other substances.
She can use a mild steroid
cream, but the main factor would be to identify the causative agent and remove it.
Please meet a dermatologist
if it persists.
Let me know if you have any other doubt.
you can ask a direct question to me on this forum, following the below link.
Wishing you a good health.