In The Guermantes Way: What makes Saint-Loup angry?

I just finished Chapter One of The Guermantes Way and I am confused on what happens that changes Saint-Loup's attitude towards the narrator from positive to negative.

Specifically, just after the narrator's grandmother starts feeling poorly, Saint-Loup writes to the narrator and ends the letter with the following passage:

"But I should not be speaking the truth were I to say to you, if only by preterition, that I shall ever forget the perfidyof your conduct, or that there can ever be any forgiveness for so scoundrelly a betrayal."

What betrayal is Saint-Loup describing?

Any info is greatly appreciated.

-- M1001

Never mind. I found the answer explained in Part Two. I thought the cause of the anger was explained in Part One and I missed it :).

-- M1001