From Longman Dictionary of Contemporary Englishon your/somebody’s mindon your/somebody’s minda)THINK ABOUTif something is on your mind, you keep thinking or worrying about itHe looked as though he had something on his mind.Sorry I forgot. I’ve got a lot on my mind (=a lot of problems to worry about) at the moment.b)if something is on your mind, that is what you are thinking aboutShe’s the type of person who just says what’s on her mind. →mind
Examples from the Corpus
had something on ... mind• As if he had something on his mind.• She had the impression that he had something on his mind and was uncertain whether or not he should speak of it.• Shiona sensed that he knew she had something on her mind.