My boyfriend is going on a pity date with another girl?

I've been dating my boyfriend for almost a month, and his school's Sadie Hawkins dance is tomorrow. On Tuesday he told me that my guest pass for the dance got lost when he and his friends were messing around, I said it was fine and we could just hang out that day. Then last night I was on Facebook and this deaf girl that goes to his school had a status that was "Snowboarding with Justin and Garret tomorrow, then going to the school dance with best friend Garret:))))))" I texted him about it and he said that she didn't have anyone to go with and everyone she asked said no and he was going to be there for his friend. I feel totally disrespected, not that he needed my permission, but it would've been a sign of respect if he would've asked how this made me feel. He's telling me it's not a big deal and that I'm overreacting. I don't care if it's a pity date or not, he's going to a school dance with another girl. It hurts. I don't know what to do, we haven't talked since I found out, he texted me "hey..." and I just said "Yeah?" and he was like "nvm then." What should I do?