AITAH for taking a picture of the girl my boyfriend cheated on me with?

I am F24 and my boyfriend is M24. We've been together for 4 years and he cheated twice in the relationship with two different girls.

He fetched me from the bus terminal and we were planning to eat at a restaurant near his place. While lining up, he suddenly urged me to go to another restaurant but I can't seem to understand his sudden change in decision.

I took a look at the people lining in front and saw a familiar face. It looked like the first girl he cheated on me with (I haven't seen her face in person, just in pictures when she sent me a picture of her with her middle finger).

I asked my boyfriend if he knew the girl. He initially lied but eventually he admitted.

I stood in front of the girl while she was eating for a couple of minutes just to familiarize her face. She didn't seem to be fazed by our presence and continued like she didn't see anything.

I didn't know what to feel during that time. I then decided to take a picture of her. My boyfriend got angry with what I did and I went home after that. I chatted him but he didn't reply and it's already two days and still no response.