Oh, My!! they are preparing to fire you on the basis of your illness. The bogus "write up" was their intitial move toward firing you. Yes, it is as illegal as h ell, but businesses do it all the time. You need to get an disability rights attorney involved as soon as possible for some advice. The attorney make be able to put them on notice that their actions are illegal on a federal basis - the American Disabilities Act, ADA.
Here is a Health Page on the ADA:
http://www.medhelp.org/health_pages/Multiple%20Sclerosis/Purpose-of-the-Americans-with-Disabilities-Act-ADA-for-Employer--Employee/show/539?cid=36
You have the right to write a rebuttal to the "write up" they gave you. Write the actual story and include the note from your doctor and insist that it be attached to the write up in your employee file. Make a copy of the "write up" and of your rebuttal for your own file and for the attorney that you need to contact.
Begin a journal in which you write down every odd thing they say to you or write to you. In this note the day and time of all things that they say or do regarding your illness. Quote people where you can. The reason for doing this is to document "in real time" any discriminatory actions they are taking. Later, if this ever comes to a legal setting, like a lawsuit, your notes will stand as a document of the truth if, and only if, they were written down at the time things happened. A diary like this can save you.
Another thing you could do is request to be placed under the FMLA - Families and Medical Leave Act. this was put into place for exactly the reasons that you are suffering now. If your company has 15 or more employees they are held to abide by this. You can request that your employer allow you time off for illness and time off for medical appointments and testing. You are allowed something like 12 weeks of FMLA time per year. It can be taken as a lump sum, as reduced hours or sporadically as time needed for appointments, testing and attacks of illness.
An important part of the FMLA is an anti-retaliatory provision that forbids you from being penalized (like demoted or fired) for your illness, family needs or disability. That means they cannot fire you for disability-related stuff while you are covered. That also means you must fight every time they write you up.
I would say that you need to take action immediately to save your job, as they have shown that they are willing to behave illegally with regard to your illness. And they have already taken the first steps. Do not worry that you do not have a diagnosis. This is not required for someone to be protected by either the ADA or the FMLA. You do have rights and it is important to invoke those rights before you get fired.
I recommend that you download and print the application for FMLA tonight. Fill in the short part that is for you. Write a short note that you are requesting coverage under the FMLA and will have your doctor's form completed for their records. As soon as they receive your FMLA notice they cannot move ahead with firing you. In essence, you tie their hands. Let your doctor know that they are preparing to fire you and that they rejected his prior note. Ask for his help as soon as possible on the FMLA form.
Here is the site for the ADA - read on it to learn your rights:
http://www.ada.gov/
Here is the form that you need to print and give to your employer:
http://www.dol.gov/esa/whd/forms/WH-380-E.pdf
Also write a short letter to your employer and state that you wish to invoke the Family Medical and Leave Act, due to your illness. Tell them you have given the form to your doctor for completion. You wish this coverage to begin immediately.
Then give this note to your boss before anyone has a chance to speak to you in the morning. This will give you some breathing room.
If your employer has fewer than 15 employees you might not be covered.
And contact a Disability Rights Attorney. ASAP. You can't wait on any of this.
I hope others, that are more knowledgeable on the topic will jump in also.
I hope this helps.
Quix