I do not believe this is a common outcome of mastectomy, but it is certainly possible. When surgeons do lymph node dissections after mastectomy, they are operating in the armpit. Lymph nodes are indiscriminate, rubbery bits of tissue that can have great resemblance to bits or fat, or even sweat glands. The more tissue they remove in the search for lymph nodes, the more likely they removed sweat glands as well. Make sure to let your surgeon/doc know during the next follow up. Or call into the question hotline if you are getting concerned.