I think she was introduced after Gus died and she let Walt know that Gus was part of a larger network who she represented. So the implication is that she was always there in the background since Gus supplied her Walt's product.

As for her being nervous she was more exposed after Gus left. That's why she was so nervous. With Gus she could stay removed from the dirty work. It makes sense that he'd work with her. It expanded his business exponentiallly and it was a more structured way to move product. Working with her was less about her and more about expanding globally...which is a total Gus move.

In BB Mike already knew her. They met in the coffee shop and discussed his guys in jail.