They cannot get your actual medical records, but they are allowed to require phsyician or health-care provider confirmation that 1) you have a certain condition, 2) which requires certain accommodation, and 3) what those accommodations are and exactly why they are necessary. (They don't have to take your word for these things.) Provide them this; if they won't grant the accommodations or want more information, contact your state department of labor and/or equal/civil rights agency for assistance.