Please help diagnose cause of death (Platy)

[Mettafin]

Greetings,
I have Platys and Rasboras in my tank, which is well established with plants. Water quality not a problem.
Two Platys recently had babies in a smaller section of the tank divided off by a screen.
I moved one of the females back to the larger, main part of the tank once she had finished having her babies. But when I saw the males harassing her, I moved her back to the smaller (female-only) section.
A few days later (i.e. today), I noticed her in the plants at the top of the tank with a large (dilated?) red anus. I watched her closely. As I researched possible causes of the problem online, her condition became worse and worse before my eyes. Within 2 to 3 hours, the diameter of the red anus had increased and looked very painful. She began hanging upside down (vertically), and at one point dashed madly around. Then she stopped dashing, hung upside down vertically again, and soon died.
I feel terrible about this because I'm quite sure she was in pain and I couldn't do anything for her. It seemed to happen so quickly.
My research turned up a number of possible causes:
-Dropsy (caused by stress?)
-prolapsed anus
-inflammation of the intestines
-constipation
Would any of you have experience with this? I attached a photo.
N.B. A few weeks ago I treated my tank with API General Cure because of a different fish that had what seemed like Wasting Disease.
Thanks in advance for your comments.
Mettafin