AITA for showing around the pic of her boyfriend?

I 18f went into school today to collect my physical exam results and me and a big group of friends who were also there for the same reason met up and started walking around the town after.

This girl im good friends with, gave me her phone, she’s letting me go through her and this other girl she fell out with messages. She gets a snap from her boyfriend and I open it with the intention of snapping him back a picture of me. I know not everyone here uses snap, if you have someone else’s phone it’s normal to just start snapping people yourself. Or idk I see it a lot anyway it’s a done thing.

I obviously didn’t know this but they were kinda flirting about what they’d be doing later which crazy to do when you’re in a group of friends. I open his snap and not joking it’s him sitting on a chair in front of a mirror naked but holding his exam results paper between his legs to block his dick. I burst out laughing and show her and she starts laughing and tells me to save it in their chats. Then she takes or back to reply for a sec and she gives me her phone back to go back to what I was doing while she literally starts talking to me about the artsy/cute pics like that he sends regularly.

I start to show some of the other people the pic just because it didn’t seem that private the way she reacted to me seeing it and thought it was funny. She IMMEDIATELY flips and starts going off did I say you could do that did I say you could do that why are you doing that what’s wrong with you etc. I say right okay I didn’t know she doesn’t stop and says most people would know how wrong that is etc. like 0-100 sooo fast I don’t get it but maybe it was that bad idk.