Hi,
Suspicious mechanic is triggered whrenever the night ends and you:
- didn't clean cum stains
- left her undressed
- let 4-eyes see her naked
If any you the above happen, she get suspicious and you cannot give her any items other than gifts bought in the shop and also daylight shower event will not happen.
The solution is simple, do your best to prevent those three points from happening during the night, if you actually happen to stumble and at least one of them happen, then you'll have to literally skip few nights and give her normal gifts or wait for the next loop reset.
Hope this helps,
Cheers