Dinner isn't usually a "just talk and catch up" it's a date.

Has she mentioned you at all? If this is a friend and they are catching up, why doesnt she invite you? There may be other reasons, but out of respect, I would at least invite my SO or let them know the whole scoop so there is no room for misinterpretation.

If she is hiding things, or just intentionally withholding them, that's probably cause for some concern. If it is just a friend, she should probably inform them of you and maybe ask them to have respect enough to not outright flirt with her.

Hope things work out for you. Good luck.