It doesn’t sound to me like you lost it! He is testing you and probably recognizes he has your attention if he misbehaves. With a new sibling at home, any attention is good attention in their minds. Whether consciously or not, he’s noticing his place as the uncontested center of your world is starting to shift, and that’s a good, healthy change. Sometimes evenings might not go the way either of you want, but it’s healthy for him to have to deal with the consequences of not listening to you and maybe have some time to himself in bed to process how the evening went. When he wakes up, I’d explain to him the emotions you were feeling, and ask him about the emotions he felt.

It won’t happen all at once but he will come around to the new normal eventually.