Issues. . .
Hi, RP! I may be new here and having some issues myself, but maybe I can offer perspective.
If I read correctly, then your major issues are:
1: Your own burgeoning jealousy
2: Her seemingly opportunistic approach to polyamory.
3(?): The fact that it sounds like the two of you may not be in an appropriate space to approach a poly situation.
Is this right?
Well, I think the only real way to progress is to examine your motives.
If you are jealous, why are you jealous? First, generally, and then specifically, in this situation. Sometimes when you drag the source of a feeling like jealousy out into the light, it withers and dies due to being kind of nonsensical. REALLY REALLY take a good look at yourself, here. Don't judge yourself or anything, just look long and hard. (hee, hee! "Long and hard!")
She needs to look at her motives carefully as well. She may need prompting from you in this regard, as it may be sitting in her blind spot right now. Why would she suddenly change her mind about poly as soon as a paramour floats along? Is she REALLY interested (and inclined to) a poly lifestyle? Or is this opportunism masquerading as such? This can be touchy, so be very gentle when bringing it up. No one ENJOYS being told that they don't understand their own motives.
I think that a lengthy discussion between the two of you (and possibly even "Ace," if things get that far) is due right away and a little soul-searching is probably necessary.
Illegitimus Non Carborundum