In how long of a time period have you lost this weight? You said it stated when you began doing XC this summer, but for how many weeks?
You may need to look at your caloric intake. How physically active were you before you started doing XC? What is the miliage you are doing right now and what types of workouts are you doing? If you were sedentary beforehand, doing XC and failing to increase caloric intake would lead to the weight loss as you are describing.
Like the poster directly above me said, RED-S would be the reason why you should be concerned. You may be ok now, but do not try to lose any more weight. Losing more weight as you are already very thin, would likely get you sick and injured rather than faster due to lack of sufficient calories and nutrients.