Called Date off because Girl had Hangout with Guy After

So, this girl and I arranged to go out tonight, but last minute, one of her guy friends asked her to hang out with him- to help him write one of his assignments for school (according to her, they do this often). Apparently, this guy played around, mentioning a pizza party at his place to her last night, but it turns out his "party" would consist of her, her sister, and him. Basically, it seems that he used the guise of a party and pizza to just ask her over. She agreed. I told her that I didn't feel like going out anymore, because she planned to hang out with another guy after. To this, she became angry. I was disheartened that she so willingly went ahead a schedule a meeting with another guy immediately after our supposed appointment. Was I being too big of a baby tonight?

If she's your girlfriend then you have the right to feel that way. If she's not then she has the right to feel the way she does.
You may feel hurt but for her you have no right to get hurt because you're not together yet.

If she was so into you, she wouldn't of made plans to hang out with him that night too, so that hurt your feelings a bit, which is totally normal. She seems like a player, watch out

Your better off not dating her. If hes causing issues so early, imagine what it would be like if she was your gf. I have a few rules when dating-stay away from people who have close opposite sex friends or people who just got out of a relationship or people who are still friends with an ex.. works for me