First I want to start by staying that I am a person who deals with an anxiety/panic disorder daily and this doesn't resemeble any problems I have had...but that does mean that isn't what it is. Has your gf ever seen a doctor for this matter. To me is sounds more like a bipolar disorder they often have extreme ups and downs and can change rapidly. I would suggest you talk with her and have her make an appointment to see her family physican.
Hope that helps!