Can't help with this Sylverclaws, but, OMG, how strange! I've read a lot online about guppies and endlers, but never read anything even close to something like this! I can't wait to hear what others say!
She was one week old, 150% positive. lol You know me, I keep track. Mom was born in a twenty gallon where there were no other fish but her siblings and mother since I made it a temporary birthing area JUST for her. She was removed at ONE week old from her siblings. There is no way for them to breed so young, the males are still female at this point and females are still young and undecided as well. They can however, play at mating after a couple weeks, but they cannot conceive nor inseminate.My guess would be she was probably stunted at birth with that spine, could have been much older than you thought, and actually mated with a male before you separated her. (Stunting in stores is very possible, and who knows the age when they get them, so perhaps she was born before you think, even though you say it was in the litter of the pregnant fish -- maybe she hid while nobody was looking). I have heard that guppies a few weeks old can try the mating routine... about equal to cloning, huh?
So if this happened, she was carrying all the time that you have had her, in that single fish tank. Seems like the most plausible possibility.
But if she has lasted this long, then she is like a slightly deformed person, can make babies, give birth, and survive it all! Good luck, she needs your love, too...
I had two German Shepherds that were black/tan. Over half the litter of 10 were solid black. That means BOTH parents had to carry the all-black gene.