Body Odour

My 12 year daughter is in good physical condition and has very good eating and hygiene routine. Still she has a peculair body odour. We can start noticing it, 3-4 hours after she had taken bath. It is quite mild though.
But after, say, an hour of tennis lessons or a session in gym, this body odour becomes quite strong. She also feels that otherwise, she sweats lesser then normal. Please advise what can be the probable causes and how can we try to help her overcome this problem; without her becoming too conscious of this state. Thanks.