What defines a friend?

I belong to a friend group. There's 4 of us. The other 3 were friends before I met them. I moved to my new school and they let me into their friend group which I am forever grateful for because I was very introverted (still am but it was WORSE back then).

But today I was swiping through my stories and I saw that they had hung out together. They went to a nice garden to take pictures for graduation and they watched a movie afterwards. And then they had sushi (that was the most hurtful because I'm a crackhead for food). And I was never notified of such a thing. Not even in our group chat. Not even asked "Are you free this Sunday?" I sent a message to their story with the food "Oooohh that looks good " Yunno, kind of passive aggressive but with no ill intent. And one of them responded "Yes it was!! We went out to take pictures for graduation. We didn't ask you because we know you already took yours."

And that's the messed up part. She could have just stopped at "Yes it was!" But then she felt the need to explain herself because SHE KNEW I was hurt by this. They knew and they still did it. And secondly, one of them already had their grad pics done and yet she was still invited and went with them. Third, they planned all of this most likely, which means they have their own private group chat. That's fine with me, I had always suspected it. But it kind of hurts sometimes whenever we're hanging out and they have their own inside jokes and I just smile and pretend I get it. And it kind of hurts because what is it that they feel they need to hide from me? I feel like a jerk for being paranoid and suspecting them but I've always had trust issues and I can't help it.

Right now I'm just overthinking about what I could have possibly done wrong. Did I say something cringy? Am I too introverted for them (all of them are extroverts)? Were they afraid of me ruining the mood because of my introvertedness? I've never talked behind their backs so I know for sure it wasn't something I said.

Honestly I don't know anymore. I'm not hurt that they went out and hung out without me (maybe just a little hurt cuz of the sushi)... I'm hurt because this just confirms all of the paranoia and suspicions I've had. To be fair, they were friends first and they'll probably always have a stronger bond. But I just don't know if they even see me as a friend or if they see me as the girl they took in and now they regret it cuz it's just too much trouble.