How do you know when someone is actually mad/upset with you or something is going on with them, but they keep saying nothing is wrong except you keep getting a vibe that something is definitely wrong?

Like they say they are fine and have moments where they seem fine but then other moments have you questioning if they really are okay?

I just feel it but I feel crazy since I keep getting the “nothing is wrong!” Response.

I usually I just drop it because everyone hates being asked repeatedly if they are okay but I get this feeling day after day.