This definitely isn't any symptom of pregnancy/joeys I've ever heard about nor experienced in my glider moms.
I did have a rehome that came to me with one flap droopy and stretched out like this, and it was to a pinch injury that caught the patagium (my vet figured the previous owner must have caught it in the cage door or something as there was bruising but no open wound). Usually gliders can control the flex in these membranes to sort of "suck" them in when they're running; she shouldn't be stepping on them. This is not normal.
Another vote for a vet check. Gliders are pretty good at hiding illness until it's too late, so any changes like this should be evaluated promptly, especially if she has joeys in pouch... Nursing two joeys is taxing on the mom, so if she is injured or sick, it's quite likely she may pull her joeys due to the stress.