There's a big difference between seeing somebody who's started seeing a few other people at the same time versus seeing somebody who might be going back to an ex. The first is somebody who is still checking out their options. The second is somebody who's built a full blown attachment with another person and hasn't let go of it yet. You can't compete with something like that. She really shouldn't have been dating other people at all if that is where things stood. You should end things with her now before you get even further into it. Let her know that she can contact you to see if you are still available once she is over the ex. But if you stay while she continues things with him, things will only get more complicated and miserable for you.
“This planet has - or rather had - a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movement of small green pieces of paper, which was odd because on the whole it wasn't the small green pieces of paper that were unhappy.” ― Douglas Adams, The Hitchhiker's Guide to the Galaxy