This patient support community is for discussions relating to Dysautonomia (Autonomic Dysfunction) including: Postural orthostatic tachycardia syndrome (POTS), neurocardiogenic syncope, mitral valve prolapse dysautonomia, pure autonomic failure, autonomic instability and others.
There's also instructions in the Health Pages up above on how to do your own tilt test, preferably with a doctor nearby, like your primary care doctor. My brain is blanking, though, and I can't find the link!
It would help to bring this info to your cardiologist if you do get unusual results. Then he'll have something to follow up on.
http://www.medhelp.org/health_pages/Neurological-Disorders/Further-Reading-on-Dysautonomia/show/696?cid=196
http://www.medhelp.org/posts/Dysautonomia-Autonomic-Dysfunction/Medical-Records--Test-Results--and-Charts/show/1044052
If indeed there weren't a bunch of episodes of sinus tachycardia (heart rate greater than 100 bpm) that wouldn't be accounted for by physical exertion on the holter monitoring, then I think you can rule out POTS definitively (unless your resting heart rate is less than 70 to begin with). If your resting heart rate (awake, don't count your sleeping one) is under 70, that's a different can of worms, because that would lower the bar for a diagnosis of POTS for you to under the technical definition of "tachycardia."
To learn more about POTS, you may want to read some of the basic articles about POTS here:
http://www.medhelp.org/health_pages/Neurological-Disorders/Further-Reading-on-Dysautonomia/show/696?cid=196
If it's not cardiovascular in nature, I would consider some of the things Surgimenopause suggested ... I would probably hedge my bets on seeing a Gastroenterologist next.