You've probably seen her once before in her life, you just don't remember. Trust me.
The other thing however, is that her personality might be totally invented. Your mind has a tendency to take civilian faces it doesn't recognize, and then plop personalities and identities in them however it wants to. After all, a personality is not so difficult to create as the sculpture of a face, right? That's why totally invented dream characters usually have very one dimensional personalities. People in the crowd serve no purpose but to be there to make the place feel lifelike. Perhaps if a giant meteor falls from the sky, they'll run.
But let's say a person runs up to help you.
If they've got a more round personality and are fully characterized, chances are they're someone you know well.
If they're more flat as most dream characters are, then they may just be a dream character established to save you and that's it. Your mind may make up their other traits on the fly.
Anyway, your dream girlfriend was probably a slightly characterized version of someone you've seen in your life, but was so insignificant that you only remembered her face and had that to work with. Maybe you don't even remember her face, but somewhere in there, you're subconscious does. From your description, the only real thing she seemed to think or convey was her love for you. That would be something I would regard as a one-dimensional character. Someone who only has one clear goal and only acts in ways regarding that goal. With an unchanging personality.
Your dream may not have had any cardinal point so much as it was just exhibiting the satisfaction of finding a lover, and more importantly (amirite?), rubbing it in your ex-girlfriend's face. Most dreams, believe it or not, do not have a real point behind them. Your mind is no more of a subconscious philosopher than you are. They're all simply fanciful thoughts. The ideas of dreams having actual subconscious meanings is a little silly, in my opinion, but some people choose to believe in them, and I don't blame them.
[TL;DR: Your dream character is probably just another random dream character.
You've seen her before but probably don't remember.
Your dream probably didn't have one massive, overarching point.
It was probably more about just having someone to love, and how enjoyable that is.
If you want to dream about her again, then that's where lucid dreams come in.
Otherwise, you've probably seen her in the past, and you may or may not see her in the future, so don't get too hyped on that idea.]