Embarassed, Ashamed

I feel like an awful parent. I don't want judgement please. So my 4 year old pointed out to me that she's caught her dad and I doing the deed. She knows the word, and really honestly I don't know how because I've never said the word SEX around her. I try not to talk in front of her about anything or make sure if I say stuff about sex it's not in front of her. She is curious about what sex is, and thinks that her humping her toys like she's been doing for months is sex and doesn't understand why she shouldn't do that now. I'm at a loss I feel shitty like I don't know how to explain why not as I've previously already discussed with her good touches, bad touches, consent, and private time when she humps/self stimulates. She is 4 I want to find a book maybe to help guide me. Any recommendations? Or any advice on how to explain this subject in an age appropriate way . Just telling the basics for a 4 year old. I am so embarassed.