First, the worries are going to make things worse - so first you need is to accept that you're worried right now, then tell your worries that you will be fine - you are a healthy human being, you exercise a lot etc .. and then immediately distract yourself... practice deep breathing.. and trust me :) worrying too much is the worst thing.. live in the current moment and happiness... when you feel down/worried think of your parents and your friends.. call them talk to them..
First, you're 15. This is kind of what being 15 is like, so maybe this isn't going to last forever. Second, you need professional help from a psychologist to find out why you're so insecure. Never ever Google symptoms -- symptoms don't tell you anything about anything on Google. Now, if you ever get a diagnosis from a professional, Google can help you learn more about that and whether your doctor seems to be leading you in the right direction. Doctors seldom tell us much. But every symptom you can have is.a part of a collection of symptoms that could be part of thousands of diseases you don't have. Stop doing that, you can't learn anything that way. You also sound like a very young 15 year old, so know that learning about the world around you is very important. Education is important. That includes health and sexual education. Ignorance isn't bliss, as you're finding out. Also know that other people don't really care that much about you unless they know you well. They aren't spending their lives watching you. See a therapist and figure this out.
This started for me about 4 weeks ago the exact same thing. If my arm hurts I instantly think the worst of it. I get a bad headache I just instantly think brain tumor. First time it happened I got a panic attack and I was rushed to the hospital it is really bad and ruining my life as well. I am 20 years of age and I am having trouble going to work. I got pains on and off and I just long for the day to end but then the next day is just as bad. It's really nerve wrecking and being stressed causes muscles aches and tension which in time causes me to panic even more. You're not alone. It really sucks.