Headaches and migraines are a known post treatment phenomena which some doctors associate with the treatment and some don’t. It could be a result from the chemo and other parts of the treatment or a combination of other biological processes (such as puberty, hormones, eyesight etc.).
It is recommended to consult with a pediatric neurologist and always stay on alert to make sure these are only headaches and not something else which may develop over time under the headache ‘noise’.
A good book to learn, assess, and help treat migraines is “The Migraine Brain”.