(((((Dear Kelly)))))
First off, I am sorry you are having a tough time these days.
I suffer horribly from PMS. My doc tried me on low-dose Zoloft, and that helped. There was some initial weight loss, then stabilization of weight. I didn’t like the side-effects, so I stopped taking it. They do have some good treatments for PMS now some of which I think you can take just at that time of the month.
Regarding the dx: if I were in your shoes – I would be angry.
Was your mother paying the T (sorry to sound cynical, but the T may have been influenced by who was paying for the sessions).
I don’t see how a “real” dx can be made from an e-mail and one session. BPD is a serious disorder, to arbitrarily label someone with BPD after one session? To me, that smacks of extreme professional irresponsibility.
Kelly – your mom’s behavior - this is what Ns do. This is how N parents act. It is always your fault, you are always the one with the problem – of course it couldn’t be them

or a mutual disconnect?
Crazy making, diversionary tactics at their finest.
Reasonable people will try and work things out with you. She isn’t trying to work things out, she is labeling you based on a cracker jack box diagnosis.
Armor on, mirror up – she is projecting on you – let it fly back to her. I think it might be a tactic to unbalance you so that she doesn’t have to let go of the control at work?
If you are unsure and want to make sure that the dx is not accurate (although I don't think you need to do this) – seek out an independent 3rd party who is expert in BPD (I might be tempted to do this just to set your Mother and the T straight.)
(((((Sending cyber hugs, a heating pad, some vitamin B, and a nice tall glass of wine [if not wine, some warm tea?]))))))
Peace