I'm going to look up my (ex) girlfriends activities online and confront her about it
Why would you do that to her?
I want to find out if she was seeing someone behind my back.
That is your ex. You should keep your nose out of their business.
I'm going to talk to them instead.
If you talk to them in a productive civil way, that is better than cyber stalking, so I commend you for doing that.  Just ask yourself if it is worth it in the long run to know.