How often and how much are you feeding? From the pictures it looks like she is just starting or fat. Chances are though, she is pregnant, as the previous poster stated. Continue watching for a gravid spot. When you see that and it looks like she swallowed a box then the fry will be around shortly after.
If you plan on keeping the fry start preparing now. Do your homework and get the required supplies. Platies will eat their young so get plenty of plants they can hide in or a breeder box. Do your homework and you'll have little fry in no time.
Sent from my N861 using Aquarium Advice mobile app
Freshwater Fish keeper with MTS currently in love with Platies!