The girl is suffering from osteopenia.
The osteopenia is a condition, in which the bone mass of the person is significantly reduced. High amount of training and less intake of fats may cause a decrease in the hormone estrogen in young girls. this decrease in the hormone may lead to the irregular periods as well as decrease in bone mass, which eventually causes osteopenia. the osteopenia is a precursor of the osteoporosis, the osteopenia later, if untreated causes a further decrease in bone mass and porous bone (osteoporosis).