My Cat Has A Swollen Abdomen And A White Milky Discharge From The Anal Region. What Could Be Wrong?

If your cat has fats hanging between legs then the cause of abdominal swelling can be obesity. Another condition which causes abdominal swelling in cats is pregnancy. If both these conditions are not present in your cat then your cat require vet visit because swelling can be due to serious problems like growths in stomach, heart and liver diseases, internal bleeding or feline infectious peritonitis.
FIP or feline infectious peritonitis is a fatal viral disease in which fluid build up and accumulate in the abdomen and chest with diarrhea, fever and loss of appetite. So, take your cat to local vet.

Along with getvet said it could be parasites so a vet visit is needed.