What Causes Fainting Followed By Amnesia Episode?

hi doctor,

my brother is 14 years old he was dignosed with burkitts' lymphoma last year and he completed the treatment protocol,and to evaluate the treatment the did for him a nuclear scanning as they found a subclavicular mass the dignosed it as lymphoma,they said that all the investigation came out normal,now he is experansing a fainting followed by amneisa episod?

so what may be the cause of it?

why it is happening?

and is it related to brain tumer and if yes why everything is normal?

please i need an answer

sarah