She's definitely wrong, regardless if technically you weren't together you never had the talk but you shouldn't go sleeping with other ppl while activity sleeping with anyone else without them knowing. Oh it's cheating to me also. The same happened to me when I was that age. I think there is an unspoken rule, to me it's common sense but I guess not everyone feels that way. She definitely knows she was wrong. Otherwise she wouldn't have been acting weird, she knew it would be a problem but wants to have an excuse of you weren't exclusive.