Stewart was caught having a fling with Ross' now estranged husband Rupert Sanders in July last year. The affair rocked her relationship with boyfriend and acttor Robert Pattinson.
However, Stewart and Pattinson rekindled their romance after a brief breakup.
"Kristen has tried so hard to put everything from last year behind her and she still feels incredibly guilty. She and Rob had turned a massive corner in recent months and they were both there to enjoy themselves," quoted a source.
"The last thing she wanted to do was make Liberty feel uncomfortable, either. She looked pretty upset in the backstage bar later on that evening," the source added.
Tags : Liberty Ross | Kristen Stewart | Rupert Sanders | Robert Pattinson
