give the reason for sir Percy's change of manner to his wife after their marriage.

Chauvelin blackmails Lady Blakeney into helping him to lay a trap for the mysterious Scarlet Pimpernel by threatening to have her brother Armand arrested. Percy sees the set up and for that reason he doubts his wife's loyalty. Still, in the end, she makes up for it by traveling a great distance to warn him (once she finds out his real identity).