If the doctor says there isn't a major physiological problem, then you might just have lower-than-normal blood pressure, heartbeat, or metabolism. Try getting out and active. Some Sun with free vitamin D synthesis couldn't hurt, either.
As always- None of my statements are evaluated by the FDA.
My advice is not intended to diagnose, treat, cure or prevent any diseases unless prescribed by a health practitioner. All information is provided for educational purposes only and should not replace the advice of your medical doctor.
(but your medical doctor will probably end up killing you