From Merrin's perspective, that is a fairly bizarre way to answer the question! She would normally expect, like, a list of symptoms, for her to use her greater medtech expertise to put together with all the additional information she has from her monitoring and make updates about his overall status? Pain (by location and roughly-quantified severity), weakness, subjective sense of deficits, that sort of thing. Instead he's...making a confident high-level claim about how recovered he is, with a bizarre un-idiomatic phrasing, and hesitating in the middle like he was having trouble remembering the words to finish his sentence.
She's concerned! Just going off the content of what he said, Estha seems pretty not-okay!
...He doesn't look sick, is the thing. Merrin has a really, really finely-honed intuition for that, by now, and her finely-honed intuition is cheerfully informing her that he looks great. Like, relatively speaking, her intuition doesn't think she should take him out on a strenuous hike or anything, but he looks like a patient who does not even slightly need intensive-care-resource-level monitoring or attention, and in fact could probably complete the rest of his recovery at home.
Merrin is not just going to take her gut-level intuition at face value, obviously. But it's loud and it's jarring and she's painfully confused.
(Imaginary Laeirthe from her fanfiction is...trying to raise a thought to her attention, wow that does not normally happen that unwilled, she wasn't trying for a Laeirthe-input and normally she has to try...the incomplete wordless mental motion is something like PAY! MORE! ATTENTION! Which would be great advice, and all, except for how Merrin does not really have any more attention available right now.)
It won't help to look stressed or alarmed. It won't help to be stressed or alarmed, in fact, and Merrin is throwing all of her years of hard-won skills and mental techniques at not being.
"Estha," she says, slowly and carefully, "do you - remember - what happened to me four months ago? ...Might've been a different time interval for you, but - after the last time you saw me?"
Something is wrongwrongwrong. Merrin isn't sure if the problem is within her boyfriend's brain or, like, with Reality more broadly, but something is wrong, and "does Estha remember the ALMOST MAXIMALLY MEMORABLE event of his masochist girlfriend's True Death in a plane crash" is a cognitive check that she should perhaps have thought to ask him first.