Ok, just a little pre story.
My gf went to this college town and she saw one of her exes. Later on she would tell me that she did love him, but anyway, he cried and explained that he wanted her back. She said she didn't want any part in it.
This... is not going to be pretty.
So we're sleeping together, and in the middle of the night I get up and she calls me his fcking name. She says "Dennis?" In getting up I woke her up.
To be fair - people say a lot of weird things in their sleep. That said, I've noticed that a lot of recent and 'traumatic' or significant events tend to be expressed in that state between fully asleep and awake.
For example, when I stood night watches during boot camp, we had guys who would sing cadence in their sleep; our recruit petty officer in charge would call facing movements and roll over in his sleep (funny). Others would start to masturbate while asleep (awkward as hell). Some would say some girls' name (presumably their wife or girlfriend - perhaps a love interest).
Anyway, I asked her why she would call me that, and she says "i really don't know", and I asked her what all happened with her and her ex and she said he cried and blah blah blah. But later that day I told her to be honest about everything and she said she stayed at his house with her friend.
Not a wise decision on her part (staying at his house).
That said, at least she is talking to you about it. That's a good sign.
What do you think it means?
I think it means that it was a significant event for her, and probably troubling.
Your reaction to this will not, in and of itself, 'make it' - but it can certainly 'break it.' If you start letting paranoia get the best of you, it will be over.
Personal experience speaking, there.
Ultimately - you have to weigh your instincts to support this girl you love against your instincts to be territorial. If you want to love and support her, then do just that. This one incident, alone, is not worth dissolving a relationship over. Unless you're not going to be able to keep paranoia at bay. Some paranoia and jealousy is natural - it's instinctive. But there is a toxic amount.
Basically, if it reaches the point where you cannot trust her... then you need to let it go.
Think it sparked some emotional shit in her and she misses him?
Well, you know her better than I do, most likely.
Would she get into a relationship with someone she did not love and care about?
Before you get wonky over that - remember that she did get into a relationship with you, too.
If I were to run into my ex, there would be some very powerful emotions involved (I suspect on her part, too - though she'd probably try to play it off). Regardless of how the encounter went - it would open up the flood gates on a lot of very powerful emotions.
Currently - I'm single, so the idea of repairing things to a point where the potential existed for us to 'try again' as fully independent adults (not a status we enjoyed in the previous attempt, and a source of considerable strain) would be appealing to that overly optimistic and romantic imagination of mine.
However, there are other females I care about quite a bit - many of which I am open to developing a romantic relationship with. The hardest part about encountering my ex (and one of the reasons I've decided to have as few of those as humanly possible) would be dealing with the raw instinct that is there to care for and support a person ... conflicting with that same instinct I have for another.
I wouldn't cheat and I wouldn't leave my current for an ex... but it would still be the fact that I have to, in a way, abandon someone I once shared a deep love with, and still care about.
She came back to you. Unless you get information that indicates she's making return visits to him - I would give her the benefit of the doubt.
But by that same token, don't blindly walk into a situation where you're being taken advantage of. This one incident, alone, is somewhat difficult to work with. Errors of judgment happen, people make mistakes - life goes on. If this forms part of a trend, then you might want to consider letting it go before it gets ugly; but that's something only you can decide to do (well, and her).