I have a question. I have recently had to miss ...

I have a question.  I have recently had to miss several days of work over the last month. I have had back problems off and on for years but not enough to affect going to work. I had surgery 11 years ago. Recently, I have been under the care of an orthopedic surgeon. I have been diagnosed with Degenerative Disc Disease, Lumbar Spondylosis, arthritis, collapsed vertebra, and scar tissue. I have been taking medication and have received one epidural injection and am scheduled to receive another  on October 3,2008. I have also been prescribed physical therapy which I will begin soon. My question is: am I covered under ADA? I feel that my job is in jepordy. I am on half-days for the second time, and some days I can't feel my feet and my legs will collapse when I walking. So, I have to call in  quite a bit because I don't want to fall while walking across the floor in front of customers. I am a customer relations manager at a car dealership. Also, my bladder has become affected. I have trouble sitting and standing for extended periods of time. My supervisor knows all of this and has received copies of my work excuses but is becoming aggravated with the time I am missing and doesn't want me to do any of my work from home(which I am able to do). I can't afford to loose my job, but I feel their patience is at an end. Please let me know something, because if they think that I am following a course like this, there could be problems for me.