Stewart and Sanders, who met on the sets of the film "Snow While and The Huntsman", were caught cheating on their respective partners - Robert Pattinson and Sanders - last year.
Stewart, 23, and boyfriend Pattinson showed up to watch hip-hop act Jurassic 5 at the Californian music festival, only to discover Ross was standing nearby, reported Sun Online. "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. "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," an eye witness said. Ross, 34, filed for divorce from her husband of 10 years, Sanders, in January after his cheating scandal with Stewart became public.