I AM NOT A DOCTOR DO NOT TAKE WHAT I'M ABOUT TO SAY AS A PROFESSIONAL MEDICAL CONSULTATION.
With that out of the way, I don't think you should get too worried at the moment.
The body fluxuates in weight throughout the day. For example, if you weigh yourself after a lot of water and a large meal you will probably be several pounds more in weight. Firstly, you need to make sure your weight is being measured properly:
1) Make sure the scale is calibrated every time you step on it.
2) Make sure you weigh yourself naked. Clothes do not all weigh the same. Clothes can get heavier with sweat, etc.
3) Weigh yourself at the same time every day. This way you are, in general, measuring yourself within the same timeframe of your daily routine; IE, if you do it every morning before breakfast you'll always be measuring about the same time as when you last ate.
I knew a guy who was a roofer in his early 20's who swore his pants didn't fit him at the end of a work day and had to tighten his belt before leaving the work-site because of how dehydrating the work was.
In addition, you're 15. Your body is going hormonally insane at the moment. Some weight shift likely isn't a concern.
Lastly; keep an eye on it. The worst case scenario, and I say this to inform not to scare, that I know of would be colon cancer. However, at your age range the odds of it being cancer are incredibly slim. Colon cancer, even in people at risk (such as myself, which is why I know about it) is generally not even screened for until your 40s-50s. Ask your family if there is a history of colon cancer. If there isn't, I really, really doubt it would be that.
If the weight loss persists or you're still concerned you should see a doctor for a professional opinion.