Hello my name is ***** ***** I am happy to help you today. Has your employer taken steps to request a medical report to ask questions about your health and it's impact on your performance?
Hello your employer does owe you an implied duty of care to take care of your health and safety. It is not appropriate for you to be perfomance managed without your employer understanding the impact of your health on your work and potentilaly making reasonable adjustments in the event that your condiiton could be considered to amount to a disablility for employment law purposes.
I would suggest you raise a formal grievance to say that you consider that they are in breach of the duty of care and that you are prepared to give consent to a medical report for them to understand the impact of your condition on your work.
If they fail to do so and continue to performance manage you you will have the right to appeal against any decisons and potentially also to resign and claim constructive dismissal.
If you have any further questions please do ask. If I have answered your questions please take the time to rate my answer. Thank you and all the best.