She's probably gonna give birth within the next few days. Put her in a breeder net or a breeder box. My platy is like that and she has already given birth to 2 fry this morning. Here's a picture of what a platy looks like when they are about to go into labor.
Here is a diagram showing what to expect when your platy is about to give birth.
A= look at the opening infront of the anal fin, this is open and white when she turned around you can see the opening quite clearly, if the opening has something dark in it she is just going for a poo, if its white and seems hollow then she is going into the first stages of labour. this is the best sign as some fish do differ b/c of the amount of fry inside.
B= this area will stay rounded until 24hours before she will give birth, then it will go box like shaped as ive shown in the picture. this happens with almost. every fish
C= this area will look like its an intense lighter colour than that of her natural colouring it will be swollen a fair bit if she has a lot of fry waiting to come out but this is not always the case as young fish with there first drop dont usual show this so well.
D= note that the shape of her back has started to arch, this is her getting into position so the birth canal will allow fry to drop.
she will be panting but not as heavy as guppies do and she will either be quite on the bottom or in weeds or she could and most of mine done go into a reverse swimming motion with her head to the floor and her tail in the air.