My friend who has a crush for me kissed me in front of my girlfriend?

I am a very shy boy. I have a group of friend which contain some girls. One of them(lets call her S) who is closer to me propesed me one day. I gently denied her told her about my crush who is in same class but not our friend (call her P). That time she happily accepted my decision and even promised me to get my crush. Slowly I started talking to my crush but a little. I was getting closer to her. P joined our group. one day when I won a tennis tournament and informed my friend when we were at canteen. Everyone congratulated me but S just kissid me on lips. Everyone including me are shocked. I looked at P but she seemed dejected and walked away. I asked S but she just said sorry. I am not talking to her from twoo days. I tried to talk to P but she denied to talk to me. WHY S DID THIS? She had promised me to help in my relation with P but ruined everything. Wht should I do?