Why does Madame Forestier speak coldly to Mathilde in "The Necklace"?

Madame Forestier speaks coldly to Mathilde when Mathilde returns what she thinks is her lost necklace. In fact, the necklace is just a worthless piece of costume jewelry, but Mathilde doesn't know that, which is why she and her husband go through a lot of trouble and expense in buying a pricy replacement necklace for the one that they lost.
Madame Forestier, then, is getting an unbeatable bargain out of this transaction. But she doesn't know that, of course. All she can think about at this moment is the immense inconvenience she's been caused by Mathilde's not returning her jewelry in good time. Madame Forestier is more than a little miffed because she might've needed the necklace sooner. It was her necklace, after all, and she must have worn it quite often to formal events before she lent it to Mathilde.