Hi,
Yuna being suspicious of you means that she won't accept any gift or accessory from you and you won't be able to trigger certain event during the day. She becomes suspicious if you don't clean up after the nightly adventure, be sure to wipe all the cum stains and if possible dress her up.
If she happens to become suspicious, she stays that way for few days, so it's best to skip couple of nights and give her normal gifs bought from the shop.
Hope this helps,
Cheers