<nods> I agree she looks like she is pregnant. If you can have a look from slightly below her (as if looking through her to the aquarium light) you may be able to see what look a little like little black specs in the region that I have marked on the photo below. If you can see them, they are the baby fishes eyes.
From head on the same female platy looks like this (I would say she is going to drop her babies in the next two days:
For contrast here's a head on shot of one of my males (all of my females are heavily pregnant at the mo, and the females are slightly wider in profile just after having given birth when compared to the male).
All the best,
Triskele.