Immortality is something you have or you don't, where she's from. Benevenuta is a have; Lucius is a have-not. She's still not quite accustomed to having to allow for more variety in the dividing lines between individuals in this strange new world she's living in now, for what that means with regard to so many things.

Someone who expects wonders, expects explanations for them is harder to lie to. That doesn't seem to incline her to stop lying, but-- she's been doing this for a long time. When she wants to tell the truth it's hard for her, painful almost in the way of trying to flex an atrophied muscle, her instincts screaming in the other direction, and frankly she doesn't want to tell him anything right now. This would be too much like a concession, like something she was backed into a corner for, and she prefers to be the one holding the cards.

She says, “Better. Better soon. I'll get some rest and let the potions do their work.”

Bless Severus Snape, and Lucius Malfoy's own culture's clever little cures. She can point to each one that'd have her with just a bit of strain by tomorrow.

It isn't as though he's going to demand she open her blouse and show him underneath the bandage.