Slept with BGF

Hello I'm 16 n I went to my best guy friend house about 3 weeks ago n we started to talk then he push me onto his bed n started to kiss me n felt me up. Well we end up doing it but he had a girlfriend n I knew that but I live in Chicago she lives in Los Angeles, so any who things were weird for us n we promise that no one will find out. But I get a text saying his girl found out n he never wanted to talk to me ever again. I dont wanna lose my best guy friend. Here is my question -DO GIRLS KNOW N FEEL WHEN THERE BF/SO CHEATED?- or someone found out n told her? Am I to blame?