She probably saw the photo's of him with the other girl on the site... People need to understand that some relationships can be stressful, and that there is most likely more going on then we can see - Girls are emotional, and some men can be to, but for all we know. He could of had sex with that girl that night, or even other girls... Honestly all we can do is speculate, but to assume someone has a mental problem because they're upset, and you don't know why is far fetched.... These are humans with feelings, your not watching some Reality show on TV with a fake story.
Imo, she's hurt because he did something either that's true or not, but I've watched him since the start of them coming, and you can tell this girl worshiped the ground he walked on, but he didn't give the same respect in return, he's always acting like a unpredictable horny dog, that just wants to get laid, by her friends.. MY "ASSUMPTION" is he either cheated, or tried to cheat on her, and she's been hurting over it. - I don't trust this guy at all. I've hung around with many kids like him in my past, and he just seems very unloyal if the opportunity arises.
OR
She could be overly worked, and stressed, and things are just tense for her atm.. That's life, people get that way at times..