So my girlfriend and I dated for six months. Three days after our breakup I saw her flirting with some guy I know. Then on that weekend she tries to hold his damn hand in front of me, lean on him, and rub his head and back. They are dating. What. The. F.
It sucks you had to see that. Try not to dwell on it though. She's technically not doing anything wrong since you guys aren't together anymore... but if she knows you're around it's insensitive to go on like that when the break-up is so recent.
I think it'll be more helpful to focus on moving on and leaving her and that relationship behind rather than worrying about what she's doing in her life. I'd avoid her when possible.