When your gf go to another place, then she feel very lonely and she miss her X-bf, then she told you she don't love you anymore, will she come back to you when she come back to your place and said she is wrong and she love you the most??
I voted for the first option.
Probably the first option, but it's hard to say without knowing the specifics of the situation - how long she was with the ex?, how long has she been with you?, how serious is the relationship? etc.