I was a bad coparent today 😬

My daughter had a field trip today. She came home in a panic last night saying she wanted a parent to go with her. I had to work. And I thought her teacher mentioned it parent teacher conferences which my ex and I both attended. He didn’t mention going so I assumed he had to work. Step dad offered to go since he was off work. My ex freaked out on me stating he would’ve taken off work. He is livid which I guess is warranted if he truly didn’t know about it. I suppose I should’ve confirmed with him first. I apologized but he’s super upset and I feel like an A-Hole. 😔