"I'm fine" usually means "I am not fine, and I do not trust that saying so will go well." It is a protective phrase: it ends the questioning while leaving a door slightly open in case you push on it kindly.
The dynamic underneath: This is a bid for care disguised as a refusal of care. The sender wants to be asked again — but only in a way that feels safe. Accepting it at face value reads as indifference; interrogating it reads as pressure. The middle path is a calm, specific invitation with no demand attached.
Replies you can send: Invite without pressure — "Okay. I am here if fine turns into something you want to talk about." | Be specific — "You seemed off after the call earlier. Was it that, or something else?" | Offer a later window — "We do not have to do this now — can we talk tonight when it is quieter?"
Is "I'm fine" always passive-aggressive? Not always. Sometimes it genuinely means fine. Look at the rest of the message: shorter replies, slower response times and a dropped topic together suggest something is being held back.
Should I keep asking what is wrong? Ask once more, specifically and without a raised tone. Repeated questioning turns concern into pressure and usually makes the other person withdraw further.