It is kind of normal and yes you should change how much she eats. A 10 pound weight gain is a lot of weight for any size dog, and I believe Chewie is a smallish one?

Spaying affects the hormones which in turn slows down a dog's metabolism. They don't need as many calories as they once did.
How much are you feeding her right now? How much does she weigh, and what do you feel would be her perfect weight?