Can my employer terminate me if not back by the end of the 12 weeks?

I have been on FMLA since 7/19/10 due to a serious illness. I used my 10 sick days from the start. My employer is using those days as my FMLA. Can they do that? I have been under the care of several specialist. I had my first surgery 8/3/10, but what was taken out was not what was causing my problem. I was referred to another specialist. The problem was noted, but because of the problem, medication and time was what I was doing for a while. Then sent to a surgeon who said no surgery. But the problem was not going away and no medication helped. I was sent to another surgeon who agreed with the original doctor that surgery would be needed. My surgery is scheduled for 9/30/10. I won't be out of the hospital until 10/4/10. My employer sent me a letter stating If I am not back to work by 10/6/10 I am terminated as of 10/5/10. and my position will be filled as soon as possible. My surgeon says I will need at least two weeks at home recovery. Which means I could return to work 10/18/10. Do I have any rights? I am so close to coming back to work. Any advise would be greatly appreciated. Thank you, Alicia