Not trying to sound "new agey". But I'm sure it will come off that way.

Growing up I had a special interest of doing numerology readings of personalities. I had a hard time reading peoples personality so I always got multiple opinions on if the results were accurate.

Oddly enough, the name you use for yourself does have a big impact on personality. The readings I did comparing birth name, married name, and nicknames, each name would come out to a different number. The people I got opinions from would tell me that at the time of the name change they could tell there was something different about the way the person I did the reading on acted, but they couldn't place it specifically. After looking at my readings they were shocked looking back at how well they matched up with each name the person used.

So given this information I can see why your gf would feel like they don't know you or recognize who they fell in love with when using the new name.

TL;DR: names have a huge impact on how people act and think.