Early leave?

Can a doctor deny you if early leave from work? I’m 38 weeks pregnant I’m in pain all the the time and my inner thighs especially when I’m walking I can feel them luck up on me and the pain is horrible, be having sharp pains from the side of my stomach I can’t breathe when it comes, been having diarrhea and throwing up, I finally told my supervisor I couldn’t do it no more my body just can take being on my feet all day anymore, and she said OkY as long as my doctor send off a paper saying it’s okay, but my ob said to me since there is nothing medically wrong with me she can’t... but just last week at my appointment she told me ask my supervisor what they needed from her and to let her know and now I’m telling her they want her to fax a paper