What is your cat doing outside? Weight gain could be from muscle. Cats that go outside usually get more exercise. Your cat could have also found an additional source of food. If it is fat and not muscle then keep you cat inside and see how it effects the cat's weight. Figure out if it was the change in environment that caused the weight gain.

Cats need exercise to maintain their weight just like people do and they can overeat.

Sudden weight loss is a bigger concern, but you are right to question your cats weight gain. It could be a sign of a thyroid problem and could in turn put her at risk for other health problems such as diabetes.

Lower quality food sometimes makes cats gain weight. These foods have so little nutrition in them that the cat has to eat to feel full, generally speaking this also means the cat is eating more fat because lower quality foods use fat to add flavor since their meat content is so low.

I would suspect maybe a neighbor is feeding your cat. Sometimes people think cats are homeless and offer them food, or maybe somebody leaves food out for their cat, and yours steals it. Generally though cats are not over eaters like dogs, but this can be an issue for some cats.

Be sure your cat is gaining weight, some get a saggy tummy, but are not actually fat, a saggy tummy is normal for many cats and not a concern.
You might want to deworm your cat if she has a fat tummy, she could be full of worms from going outside.
I am going to assume she is spayed and as such is not pregnant, you should never let a cat who is not spayed go outside.