You're at the age when anxiety and depression come full flower for most people who are going to get them. It could be this is your time. Or, it could also be something that happened not too long ago that has caused you to become very insecure about health or about life in general; that's also common at your age. It sounds like it's time to talk to a psychologist who specializes in anxiety disorders so you can get on top of this type of thinking before it gets chronic and figure it out. As to the source of fears and anxieties, for some it's something that happened to them, for other there's no apparent reason at all, but it really doesn't matter, some people are just more prone to others to being anxious. The important thing is to recognize your thinking isn't helping you any and try to change it somehow, whether by lifestyle changes, therapy, or whatever works for you. Good luck.