I've always been in good health
, and had been very sucessful in athletic pursuits when younger.[Not so much now, due to age and various injuries.] I only bother saying this because it rules out age, injury, puberty, etc. as specific causes. Also, the first 17yrs., never drank, smoked, or did drugs. Again this is notable, because I've noticed this condition since at least 8ys of age. The key here is, I was a [good] competative swimmer, but always very anxious, before during and after events.