You may need to have a Tilt Table Test for diagnosis. Research that and be prepared. It will be an unpleasant test if you do have POTS, especially after they give you the nitroglycerin. Within 2 minutes my whole body went cold and everything went black. Wish someone had explained all that MIGHT happen before hand...I tend to do better if I know what to expect....scared the heck out of me.
I have been symptomatic since 6 years old and have had lots of EKG's, echo's, holter monitors, treadmill stress tests, etc.... everything was always fine, in spite of how I felt. I'm the one who finally suggested to my cardiologist to try the TTT and he was surprised that I got a positive result. Don't be discouraged if you have to convince your DR to agree to the tests.
Good Luck and keep us posted