"Captain Andor doesn't get to decide that." Her mouth narrows into a line, realizing belatedly that her temper got the best of her and now he knows she knows Cassian and it wasn't that she was hiding it! It's just complicated for reasons she doesn't fully understand.

Also it's annoying.

There was an unspoken 'no more secrets' when he welcomed her home, but here they are -- well, here she is -- and apparently there are secrets.