Whew that is quite a tank for Platy's! You keeping only platys in there? Off topic, i know. So anyway...she does seem to have a little girth, but I'm going to say that she's in very early stages of pregnancy, and shouldn't need to be in the breeder trap. Honestly, if you could add some floating plants to that tank she'll have some natural places to retreat. If you don't go with plants (live), plastic plants can be used, just pull them from the base, get a piece of Styrofoam, connect and let it float! I did it for my guppy's, molly's and platys so the fry could have places to hide.
I imagine you have a pretty good filter on that tank so it is possible the fry get sucked into the filter.
If she continues to get bigger and that dark spot near her vent (your third pic was great in that regard) continues to darken and get bigger, move her back to the trap in about three weeks (they give birth about once a month).