The best friend ex girlfriend rule?

im a junior in high school. & for the longest time i was talking talking to a girl i liked. she liked me and i liked her. we almost were about to get together. but she called it off and told me she didnt feel the same way. im like ok i understand blah blah blah. i think im over it and then she asks my bestfriend who just happens to look a lot like me to sadies. after that they start talking talking to. im really uncomfortable with it. he said he wouldnt get with her and right now theyre just talking. but i know she likes him and it bothers me. comments advice?