What I learned was that BPD and Bipolar disorder kind of go hand in hand most of the time. Atleast it did with my ex-wife... it was VERY hard to know who she was when I would get home or even wake up in the morning. You start to give names to the different "sides" and when you wake up in the morning and you talk to her for the first time you realize that "oh, she's such and such this morning." And yes, like you said, she would change so fast that it would leave me wondering what just happened. Odd thing was that she would be talkative, having a good time (if we had gone out to eat somewhere for example) and then suddenly she would just go quiet, just like that, just shut up for like 2 or 3 minutes and then she would suddenly be "someone else" - and usually NOT in a very good mood.