I think I posted on your other thread.
I actually started with two platies myself some years ago that came pregnant as well, a red and a yellow. One always hid from the start, she was just never looked right to me or behaved right and died a year later though she was never sick. It was the smallest of the two. I think it was genetical or the way she was raised was not healthy enough. The other one lived nearly 4 years and is the grandma of all my platy population. She gave birth a few times enough to produce males too and now I have so many I can't count.
I'd suggest just keep an eye in case she came in sick with something Watch out for flicking/flashing, thin belly, etc..A healthy platy doesn't hide but it's not necessarily a contagious disease.