I (m27) found out that my GF (f34) went on a vacation with her ex husband and kids without me knowing

I need some outside opinions, I've been dating my GF for 2 years. She has 2 kids with her ex husband. Curiosity got the best of me as there has been some occasional weird behavior and I looked through her phone. I found pictures of her and her ex on a vacation that I thought only her and her kids went on. Along with another outing a few weeks ago that I had no idea about. I even meet her the same evening and asked about her day. She said all she did was work. I feel hurt that she couldn't tell me about this and have been lying to me.

I understand that it's okay to have your ex see the kids but going on a 4 day vacation together without telling me is a huge red flag. Is this normal?