Friend ditching me for guys?

Shes in grade 9, while I'm in grade 11 she's like my little sister. She is super naive and that often really pisses me off. She hangs out with these really loser guys, she doesn't relize that all these guys want is to bang her. Yesterday, she texted me saying that we should hangout on Sunday, I said yes and we made plans. Then 15 minutes later, I texted her saying how I was stoked to chill tomorrow. She then said she couldn't. Today, I texted her and she is hanging with some guy. I find it pretty rude how she made plans with me and ditched me to hangout with someone else. Am I over-reacting or would you be pissed off aswell?