I have learned over the years as an employee, employers are always short-handed and will always need you to come in. As a manager I know, employees are always calling off and need people to fill in. It is up to the employee to honestly say when they can work, fill in when they can if they can, and stick to it. If it changes, be up front to come in more or less; if not, stick to what they can realistically do, because burnout and unhappy employees does not help anyone. It is the managers job to fill shifts or vacancies, that is why they are paid more even though some unfortunately try to push it off onto the employee. As an employee there is a responsibility to uphold what you are hired for, stick to that, change where you can, and then just be upfront if circumstances change with as much notice as possible.

I have found sticking to 'I cannot work at a certain time due to "medical reasons" ' with plenty of notice is the safer thing to do and much appreciated rather than to disclose the actual illness. I know for some, disclosure has worked out but I don't want to give any reason other than job performance to bias my position, and it has been used against me when it has been disclosed. It helps to keep things more professional as well with less chance of discrimination.