Have you noticed any fighting lately? Sometimes abscesses can be caused by a bite from another glider. Or possibly hers is from her, trying to fix the constipation that probably felt uncomfortable.
In any case we can only speculate. If you don't already have the Vetericyn spray, definitely pick some up. In my limited experience with abscesses, it seems like once a glider has one it can be more prone to having more down the road, especially if it's bacterial-related and the bacteria isn't completely addressed.
Let us know how the vet goes, hope she's on the mend soon!