Illness or Injury? Juvenile Platy had red spot and died

I'm not sure if my water parameters are needed for this particular one, but if it helps figure it out....
Ammonia and Nitrite are zero, nitrate is .5 PH: 7.3 GH 11

The tank is a 20 gallon planted tank I've had for...a bit over two years. It's been used as a nursery for the majority of that time. Temp is 78 degrees, I have gravel and a 20-40 gallon aquaclear filter with a sponge in it that has been running since I got the tank.

I change out 25-30% of the water once a week and vacuum just under half the gravel, another 2-5% daily change when I suck out uneaten food. I use Seachem Prime and Aqueon Water Conditioner to treat the water.

Currently there are about two dozen youngsters in there, platies and Endlers fry mostly, all babies are 1/2 inch to 1/3 inch or so except two platies who were an inch and about to get put in the main tank...now one...and about 12 newborn muppies. Oops. lol

I feed them Hikari First Bites, baby brine shrimp, daphnia on rare occasions, algae wafers, tropical flakes I turned to dust, and fresh greens or frozen greens like emerald entree, (I pick out the big stuff they can't eat yet and make sure it's all the little stuff), also blanched peas about once a week or so, cucumber without the seeds that are mainly for my snails but the kids like them too.

Haven't added anything new to the tank plant-wise, haven't messed with the substrate, but I often get new babies in(a new batch every month or so really lol), usually I catch them as they're born and move them over. The newest batch was born about a week ago, nothing wrong with them, no deaths, they're kept in a mesh net I made sure nobody can get behind by tightening it to the sides so it's flat on the glass.

I THINK I got all the needed info...

Anyways, day before yesterday I had no problems with any of my babies, then yesterday when I turned on the lights I noticed one of my young orange MM platies wasn't swimming up like they all usually do. I took a close look at her and her entire belly looked like it was full of blood. There are no juvenile males in the tank(none that are old enough for us to know yet anyways since all the babies are two months or younger aside from her and her sister)...but I have to say it looked like her insides blew or something. Just a big red spot where her gravid should be that spread to her back end, I'm sure she bled out but I'm not sure what happened...can parasites or some illness cause this?

Everyone in the tank has been bright and active, I do regular water tests(every other day since with babies you can never be too careful) and it's stayed stable for months. There's no big rocks or caves, the sponge on the filter helps lessen the power so it doesn't blast everyone around...nothing to hit or hurt themselves on, all the plants are stem plants, anubias, and an umbrella(so basically nice soft plats with big leaves)....How is the largest baby in there able to get injured like this? She was almost double the size of all the other babies...nobody ever fights or anything(that sounds odd, but I have seen fry get into it before, even new babies! But none of that has been going on), no nipped fins, injuries or odd marks, no bloating or thin babies...what in the world happened here to give her internal bleeding, or whatever it actually was?

I have a photo, but unfortunately I can't get it up, this isn't my comp but I'll TRY later on. I took a photo after she died, she still shows her odd red belly(which looks internal, I don't see any damage on her from the outside at all that would suggest she had been injured).