Help?? I cant stand my best friend anymore?!?

Ok, he asked me to the Sadie Hawkins dance as friends, and I said yes. But the next day this girl he kinda likes asked him to the dance, and he told me about it and asked me what he should do. I told him to just tell her he already had a date, but he said he didn't want to hurt her feelings and that he kinda liked her. So I told him that he can just say yes to her and I wont mind, which was a lie. I could deal with it if that was the whole problem, but he told her how he felt and now they're relationship might lead to more. This makes me sick to my stomach, I dont like seeing them together. So now I avoid my best friend and when we do talk I only answer with one syllable words. He hasnt noticed that somethings a matter, and I dont know what to do. I dont love him, I just feel replaced. What do I do?