"Within normal limits" really doesn't say much about the holter results. The sinus tachycardia episodes of POTS are sometimes not interpreted as clinically significant on 24 hour monitoring because healthy people will have episodes of sinus tachycardia corresponding to increased activity (i.e., physical exertion from running up a flight of steps, having sexual intercourse, straining during a bowel movement) that they might not note on their paper log of what they were doing throughout the time they were wearing the monitor. You may want to ask for a copy of the holter results for yourself. (You have a right to your own medical records, though there may be a small fee involved for making the copies. See here:)
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.