While thinner people are at less of a risk of heart disease or diabetes, it does not disqualify them. If there is heart disease or diabetes in your family, you are at a much higher risk of it. Sometimes these conditions can be hereditary, sometimes they are brought on by outside factors. At your age you should be getting 30 minutes of active movement per day. I wouldn't worry about it at this point. If you are going for regular checkups with bloodwork, if anything is irregular, your doctor would let your parents know. I'm sure everything is fine or you would have heard.