It makes total sense to me that you'd be very concerned.

I'm wondering if meeting this guy is possible? I know it would probably be uncomfortable if the three of you got together for a meal or drinks, but it could be enlightening. You might decide that there's really nothing to be concerned about.

I can't explain why your girlfriend is behaving the way that she is. I do know from experience that long-distance relationships are very difficult. Maybe you can try to figure out how your girlfriend would be behaving if you both lived in the same place.

You obviously love her very much. It seems worth taking the time to meet this guy to see if it helps your feelings any.

I hope that you find a solution.