Hi,
She becomes suspicious if the night ends (or you click "end" button) and:
- she's left naked
- she's left with cum stains anywhere
- during the night, when 4-eyes comes to check, he sees her naked (he blushes and stays a little longer)
If at least one of the above happens, she's suspicious - she won't accept anything besides normal gift and you cannot get a special shower scene.
To fix this either prevent the three bullet pointa from happening or if it already happened, wait a few nights (rumors say 3-5 nights) or until next loop reset.
Cheers