I would think the heavy breathing might be because of the extra weight. What kind of medicine is she on? I do know that some medicines will cause weight gain so that could be part of the problem, but if it's hard for her to breather, that might be making her less active which would also contribute to the weight gain. I would definitely check with your vet because if she does have some kind of condition, the earlier it's caught the better. Please keep us updated on what you find out.