I have a swollen abdomen and rapid weight gain. What could it be?

See a doctor ASAP. This should not be a presentation for online consult. Your doctor needs to ask specific questions, look at your abdomen and physically check for clues. You will likely need blood tests and x-rays for a diagnosis and treatment planning.
Fluid build up. You should seek care with your physician. Your symptoms could be ascites, an accumulation of fluid in the abdominal cavity. Another concern is growth of a cyst on a solid organ. Hope you feel better soon.