It seems ok to say something along the lines of expressing how you feel about her first, then how you personally have been struggling and afraid that it's getting in the way, and how you've both been under stress with the iguanas and how you really hope that she holds the same view about being open and honest because you care about her and your marriage, then just say how you felt the conversation went hush when you walked in, is everything ok with her mom, is everything ok with you as a couple.
Safer not to come accross combative verbally in insisting it is behind the back talk, but if unhappy with her statement, just say you were feeling kinda bummed because that's how it left you feeling and you hoped it wasn't that because you value her and your marriage and wouldn't want that to affect you both.