POTS is a hard diagnosis to make, and the mechanism is not understood very well. It affects predominantly young women, usually it is triggered by going from sitting or lying to standing position, which then leads to palpitations, dizziness and, even passing out/fainting. Things like exertion, eating and even stress can trigger theses episodes.
POTS is usually treated by cardiologists specializing in the heart's conduction system (electrophysiologists), or neurologists. Some large hospitals might even have a syncope (fainting) clinic with doctors collaborating across specialties to treat POTS. I recommend that you get a referral to such a medical center for further diagnosis and treatment.
Good Luck!
Thanks very much for your answer there, I've got an appointment with my GP next week and I'll see what they can do with referring me etc. :)