Is she involved with another man beside me

We are not psychics, sorry.
Ask her!
If she says yes, break up with her.
If she says no, believe her.
If you can't believe her, break up with her. There's no point in staying with someone you don't trust. Of course she might be totally truthful, and you are just one of those suspicious, jealous types.
All that is up to you.

My crystal ball is saying maybe, maybe not. Care to clarify things?

Only she knows. We have no way to know. So what's your purpose in asking us? Ask her! If she says no, and you don't believe her, then break up with her. Either way, break up with her, because you obviously don't trust her. Trust is the most important part of a relationship, if you don't trust her, then it's time to move on.

Most likely. Or maybe not. Probably sometimes.

Why are you asking strangers what YOUR girlfriend is doing? Why don't you know? What makes you so suspicious?