I am extremely confused in this relationship now...please help?

so my boyfriend and i...well we're not really together, we're more like friends with benefits and we both prefer it that way...i think. thats not the confusing part, though. so we were over at his house watching a movie and i got up to get some popcorn and his friend (who is almost like his sister) sort of ambushed me and kissed me. when she pulled back she looked really confused and upset, but i dont know why. i asked her about it and she apologized for startling me, and begged me not to tell anyone. now im really confused and i dont know what to make of it. maybe one of you guys could tell me what the hell happened? what should i do now?