Of course you haven't given us an idea about your food, so I guess it depends. If you are doing all that exercise and then only eating a lettuce leaf and an apple all day every day, i would be HIGHLY concerned. However given the amount of weight you have lost over that time period, I doubt that is the case (I would expect you would have lost a lot more than that if you were truly starving yourself and over-exrcising)
If it's really bothering you and you want to be sure, go speak to your GP (it is still possible to have an unhealthy attitude to food and exercise but not lose excessive amounts of weight). Your friend might be genuinely worried (perhaps over nothing, but nevertheless genuine), in which case seeing your GP might help allay your (and their) fears, and may even give you a good idea about what you should and shouldn't be doing at your age (you're still growing so you really don't want to put too much stress on bones and joints that are still changing).
Having said that, exercise is a great thing to do. It helps your moods (which, let's face it, at 14 are more than likely becoming a bit rocky just now), and keeps you fit and healthy. It also can energise you if done right, so that you can concentrate more on your school work.
35kg lost. (November 2005 - October 2006)
15kg gained again (as at October 2010).
Back to the drawing board - Let's do this thing!
"You can't change the winds, but you can change the sails"
"Reach out and take control of what lands in your lap"