How did I get here?

I don't know what  to think now.  I had my doc write me a note for work that it is too difficult for me to teach classes on top of my regular duties and that it would be in my best interest not to teach them anymore. So, my employer wrote him a letter and wanted detailed information about why I cannot teach the classes.  I volunteered to teach these classes when I was healthy.  Now that it is too taxing on my body, I can't seem to get out of teaching them.  How does that happen?  So, my doc's office called me and had me come in for an appointment so I could help them write the response.  My doc was pissed about it anyway. I told him that I thought that my employer is looking for a way to fire me.   He took a copy of a letter that I wrote and is going to use some of it. He said it is really just too difficult for me to be walking around a classroom and then do my regular job. He said that the stress from that alone is enough to send me into a flare.   
I can't believe that my employer sent a letter to my doc asking him to be more specific about why I can't teach these classes. I took them a note from him saying that I can't teach and I feel that should be enough.  He felt that way as well.  
I know that my bosses see me when I have to use a walking device to get around the office.  Do they think that  I am faking?   What else do I have to prove to them?  This is becoming quite the ordeal. I don't know where to turn next and what to do.  I feel that perhaps I should contact the ADA and find out if they are crossing the line.  
I do know that if they have plans of firing me, I will sue them.  I don't want the job back, but I want them to pay me what I would have earned for at least the next 20 years.  I am only 43 so, retirement age would be at least that far off.