That platy is indeed female, you can tell by the big fan shaped anal fin(the fin just before the tail fin starts, males have a pointed one that is thinner and shaped more tube-like, they also often hold it against their bellies but occasionally you can see a bit of a bump and what looks like a second very thin fin sticking off below it), and quite likely pregnant. It can be hard to tell sometimes since they are pretty thick fish, but that's 100% a female. I breed them so it's gotten easier to tell them apart after a couple years of it. lol
Now, with the white ones, sometimes they come with the reddish gills, other times it means it may be ammonia poisoning. Just to be safe, check your water parameters and make sure you have ZERO ammonia in there, they can be a bit delicate to it sometimes.
Other than that possibility, it looks like a normal healthy platy. =) Expect possible babies within a week or two! Though since it's a livebearer, it can hold for months, and may just be a greedy-gut, they like to eat and get chubby, so be careful of that. ^_~
This is how a males anal fin looks to give you an idea of which fin we're talking here(I say this because when I first READ it without a photo, I had no freakin idea what they meant! lol). http://www.bettatrading.com.au/image...Gonopodium.jpg