Thank you for trusting your question to JA today. I am a licensed attorney with over a decade of law practice and over 20 years of experience in the legal field. I’m happy to be of assistance.
Unfortunately, without a contract of employment that guarantees a specific job and terms of your employment, your employment can be altered at any time. This is called "at will" employment and it means that we have no actual rights to our position, to a specific shift or specific job duties. We can be removed at any time.
Now, you can certainly argue that this treatment is retaliation for your having used an accommodation for medical purposes, but you have to be able to prove that. The employer doesn't have to prove they had a good reason for the move, you bear the burden of proof here.
Furthermore, if the employer can justify the move in some way relating to business needs, you'd lose the case. On the facts you have now, I don't think you have enough to frame a case. However, you have sufficient reason (if you wish) to file a complaint with the EEOC, a government agency that is tasked with investigating these sorts of claims and finding the evidence that you presently lack.
If you have any further questions, please let me know. I invite follow up questions, so use REPLY for those. If you have no further questions then good luck going forward and please do not forget to rate my service with a top-three rating so that I receive credit for working with you today. Please rate me based on my service and not on your satisfaction with the law, which I am not in control of and I am just reporting to you. Also, feel free to request me in the future, if you have questions concerning a different matter.