Sorry that your post has not been responded to earlier, many of us here are unwell at the moment and are trying our best to reply as quickly as we can.
I can relate to your fear and anguish regarding your diagnosis and your personal situation. I am a single mum of 3 young children and did not know how I was going to deal with POTS and being a parent. But there are things that we can try to ease our symptoms.
What testing have you had done so far? There are several conditions that can cause an increase in heart rate upon standing, POTS is diagnosed after these have been ruled out and is done so by a Tilt Table Test. I would push for one of these to be done. As well as the tachycardia, chest pain, weakness, dizzyness etc that I suffer, I also have arm numbness and leg numbness on occasions, therefore my POTS dr wants to do a MRI of my brain to see if anything else may be going on. I do not think this is wrong for you to recieve one.
What happens to your BP upon standing? and what symptoms do you suffer apart from the ones you mention?
Below is a link to our health pages, they give lots of valuable information and links that may be of benefit to you:
http://www.medhelp.org/health_pages/list?cid=196