am I overreacting?/ should I put my pride aside?

So my boyfriend and I have been together for almost a year now. We are both 17. We are very open and honest with each other and tell each other everything. He's been there through so many things, especially when I've been bullied by girls in our school. Girls in high school can be very vindictive and mean. Last week a girl at school who has been bothering me, told me some really terrible things and treated me horribly. I stood up for myself but that didn't seem to do anything. I briefly told my boyfriend. He didn't say much. But he told me to avoid her and to walk away if she approches me again. This week was her birthday. She invited me to her birthday celebration ( I guess to make a joke out of it).   I told my boyfriend I wasn't going and and he said okay. He wasn't invited anyway and didn't care. Today was her birthday celebration. My boyfriend texted me this morning saying that he's going to her party because his friends want him to go. I left him on seen because now I'm mad. He says that it's not that serious. So I don't respond. Later in the day I see him on snapchat with all his friends and he's singing to the girl and I feel absolutely terrible. He knows that she isn't nice to me but yet he still went. We haven't talked all day. I want to talk to him but i have so much pride and I'm still angry. He hasn't reached out to me not once. What should I do?