First off, I hope that you will understand that I am trying to reassure you, not be dismissive of your concerns. If I were you, I would also be concerned. It is my hope that the information I give you here will help allay your anxiety, because I think if you know all the facts, you will see that there's actually less cause for concern than it first appears.
Many people focus on the heart rate increment (and sometimes blood pressure fluctuations that may be present, depending on the subtype) in POTS. ***HOWEVER***, and this is a big however which simply CANNOT be sidestepped, it is a requirement for the diagnosis of POTS that symptoms be present (as you noted, typically: lightheadedness, dizziness, nausea, vomiting, tremulousness, sweating, vision changes, cognitive difficulties, shortness of breath, changes in skin color (esp. extremities/face), tingling sensations in the extremities or face, coldness in extremities/face, flushing, near-fainting, fainting, and/or headache). People with POTS have several of these symptoms when upright; some people experience a staggering number of these symptoms, though not necessarily all simultaneously. Also of note is that most POTS patients report symptoms of the syndrome which persist between acute episodes (from standing, exercising, heat exposure), causing them to feel ill for days, weeks, or even months at a time during "flare ups."
I think that you are wise to follow up with your doctor; Holter monitoring seems quite reasonable. There are other causes for sinus tachycardia besides POTS, and your doctor will have a challenge in this differential diagnosis as it can be tricky. You may want to share this article with your doctor/cardiologist:
http://www.ncbi.nlm.nih.gov/pubmed/16028460
If you want to read it yourself, you can find the pdf of the full-text article here:
http://onlinelibrary.wiley.com/store/10.1002/clc.4960280603/asset/4960280603_ftp.pdf?v=1&t=hcgkqgx7&s=1c182e524c858dfd2b3043d9c9b7329a5ce614c8
(I hope that URL works for you ... it looks like a mess. It's working for me in Chrome, but Chrome does a pdf preview thing that other browsers don't do. If you have trouble with it, try just searching google for "Deciphering the Sinus Tachycardias" by Shamil Yusuf.
Make no mistake, I'm not a doctor and I certainly can't tell you online or otherwise that you don't have POTS or do or don't have anything else. I'm just a patient giving you my two cents that from what I understand, you probably have less to worry about than you think. I know how anxiety can be, and it's a vicious cycle where the more you worry, the more your body responds and gives you stuff to worry about! I hope you can relax more now. I'll also leave you with some information we have in our health pages specifically about the minutiae of diagnosing POTS (which can be tricky unto itself, even for docs as intimated on this health page).
http://www.medhelp.org/health_pages/Neurological-Disorders/Dysautonomia--POTS-Diagnostic-Criteria/show/1011?cid=196
Best,
Heiferly.