This is not a good situation to say the least, its never good when a ex shows up and things get tense. I wish I could tell you that this is all normal and things will work out, but I want to be honest with you that her actions do show that there is a big problem here.
While they may just be friends, the fact is this renewal of friendship should not cause her to want space. Of course she is in a stressful situation with the divorce, but she was in that situation prior then this and seemed to be handling it well.
It sounds like she may have some feelings for the ex, and may want to check that avenue out. While I know this is scary, the fact is she has the right to ask for space if she wants it.
What I would suggest is calling her and asking her if you can sit down and talk. Let her know that you love her, and that while you are upset about all this you do want to talk about how this is effecting your relationship.
At dinner, be point blank and honest. In a calm rational voice let her know that you suspect she is seeing him, and that is fine if that's what she wants.........but that you need her to be honest about what is going on so you know where you stand and where your relationship stands at this point.
While the direct route may give you answers you really do not want to hear..........the fact is it will give you the truth and you can decide where you want to go from there.
Of course you can always give her the space she is asking for and hope for the best as well. It really depends on what you want to do..........
The fact is she is likely seeing him. That's something you have to accept, she has asked for space, and that is something you must respect. But you have the right to know the truth and to make the choices you need to on wither you want to move on or wait it out.
I can tell you that her choices are ones you must respect.......no matter how much you love her and want to be with her if she wants space you must give it to her. At the end of the day your choices are to either be up front and demand the truth, or sit back and wait and see what happens. I can not tell you which route is best for you.........only you know which is best. But you must decide what you want to do and then make your choices.