After learning that Lucious didn’t actually do anything with Treasure, she wanted to come clean.
Carol talked her out of telling Lucious; notably, Carol believes that Cookie did sleep with another man, as Cookie left out the key detail that she left before any more could happen.
When he does inevitably find out, he’s not going to be thrilled that Cookie shed a layer of clothing and got to at least first base with Damon before deciding to back out, even if he might be relieved that they didn’t actually have sex.