You put the cup down on the wall and say you have to go, and walk away while she is still mid-sentence.

She calls after you once. Then nothing.

Two days later she texts: I don't know what I did, so just tell me. You don't.

You are not lying to her. You are protecting her from the inside of your head, which is a noble thing to do, and you do it by standing in a corridor doing nothing.