What might be Madame Loisel's thoughts and feelings right after she learns that the diamond necklace was only paste?

That was some pretty bad news! After finding out that her last 10 years of poverty were driven by a sham, she was in complete shock and despair. She had been excited at using the necklace that she did not bother to check the value of it. We don't get to read her actual reaction but most likely it would have been humiliation and shock.