Don't think it's a tumor or dropsy dude. fur_ducky says its ACTIVE.
Ok, has it been more than a month that your platy has been pregnant? If it is, there are few reasons, one - the temperature, low temperatures make the gestation period longer. second - age, the older the platy, the faster and larger the drop will be.
There are alot of other reason why it's not giving birth but this is what i can think of at the moment.
-Chewy-