AITA for getting upset with my partner for spending his birthday with his ex?

Today was my partner's (of 4 years now) birthday and everything had been great until he went to work and I didn't hear from him like I usually do. I got nervous and tried calling and then eventually went out to see him. I saw that his ex wife was there (they do have a 6 year old child) and he only admitted to it when I called it out. We have had issues in the past with him going to her when we've had issues, which is where my insecurity stems from. Also for context, I just had a miscarriage and things have been emotionally heavy for the past week. I haven't been feeling 100% confident or back to baseline but I have been trying to stay positive and supportive of his feelings on the matter. My partner was planning on going out to a birthday dinner with just her and their child and not telling me about it. When I expressed my discomfort with this and asked that he "please not" go out with just her, i was met with anger and told that I was "over-stepping". I insisted that he send her home, as it was not normal and it made me uncomfortable. He told me that I "ruined his whole night" and "made it about myself". He also stated that he, "didn't have to tell me anything", that it's his birthday, and that "we are not ok right now". He stands by the fact that they are "just friends", but it makes me uncomfortable that he would hide this from me and become so defensive when called out. AITA here? Am I completely overthinking the situation? Should I message the ex and ask her why she's stepping in, or just leave it be? Feeling defeated :/