well i changed some water cleaned the filter it must have tripled the water flow rate turned up the airstone and within an hour she was fine and the tank was full of babies.
Some of the livebearers (platys, mollies etc) tend to do that. I wouldn't call it unusual or anything to worry about, unless there are any other symptoms?
Well, if she is very fat and she is swimming up and down the corner of the tank, sounds like she is possibly about to drop. Does she have a gravid spot?