Lately my cat Sadie has been breathing fairly hard. I can see her sides moving in and out at a fairly fast pace. I've never noticed it before. She's gained a lot of weight in her three years of life, and she's naturally big, but I wonder if this heavy breathing is caused by her weight gain, or something else entirely? I've been debating taking her to the vet. She acts completely normal and it doesn't seem to be affecting her, but it worries me just the same.