If this is medical, shouldn't there be a medicine to keep her from screaming? Why should everybody else have to put up with it? I wouldn't want to do that to others around me... What a sticky situation.

It sounds to me more like psychosis or something by your description of her overall behavior. I think I'd also call 911 whenever it happens. At least then she'll eventually get the help she needs if it's a medical problem (hopefully, anyway.)