Can my job tell me I cannot work just because I’m pregnant and I’m order to work I need to provide a letter from my doctor stating I can work without restrictions? I never requested an accommodations. I am just pregnant.

I was asked to provide a letter of clearance advising that I can work without restrictions. My doctor was a little hesitant at first because she asked me if I told them I needed accommodations, I advised her I did not. She still faxed the note, and I have worked around two months since that fax was sent. Today I was told they cannot find the fax and I cannot work this weekend until they receive it. I looked online and it seems as if this is a discrimination issue solely because of my pregnancy. I am a Certified Nurses Assistant, however I did NOT request any accommodations because I am fine to work. Is it legal for them to tell me “because your are pregnant we need a note advising you can work without restrictions”, just because I’m pregnant?