Hi. Me (M/15) and my class were selling cake for Valentine's Day. I bought drinks and the person (M/15) I want to talk about today bought tissues and cake. Now, everyone payed the things they brought, including decoration, the cake, you know all that stuff. Now, before the sale started my classmate (F/15) messaged me about bringing stuff, but also asked for money so she can buy tissues and decoration. I told her that I have trust issues and don't want to. Now the thing is that after I told her about this thing with the money, I texted that I was fine with everything else. I didn't really tell her that I don't want to give her money, that was one mistake. Now: she bought her tissues and now wants me to give "her" money back. I think that It's unfair that everyone had to pay their stuff except her, and I also don't really think that the tissues costed 5€ (I live in Germany. Sorry for bad grammar or spelling.)
Now, she slowly started to get on my nerves and I thought "Nah it was kinda my fault. I should give her the money" I was about to tell her that I will give it to her.
Before i could tell her, she started insulting me. She said that I will never find a girlfriend and die alone as a Virgin. She started saying that people only talk to me because they don't want to offend me, so they can stay out of trouble. It really insulted me, so i just called her a slut and didn't talk to her again.

TL;DR: Girl (F/15) texts me (M/15) that she wants money for buying tissues for a cake sale. I refuse, but kinda mess up saying that. After she buys them, i refuse to give her the money, but in the end realise that it was kinda my fault and want to give it to her. She starts insulting me before i can, i call her a slut and dont talk to her again.

Now, AITA for doing that?