Er - an hours walk at nine months? What are you trying to do walk the legs off her? This is far too much, and don't forget, the weather is getting warmer all the time so she will feel it more, as well as having been a bit under the weather with the season.
Give her a couple of shorter walks a day rather than one long one, and in the warmer weather make sure its either early morning or late evening, when its is cooler. Keep any long walks until she is at least a year old, and later if she is a big breed.