Do I have a discrimination case if I was replaced by a younger worker and at a reduced salary?

I was employed by an electronic company on January 2012. I was briefly laid off and was asked to return to work. In June 2015, I had cancer surgery and the treatment continued until the end of November. I was on disability during this period and my employer was informed each time I got an extension from my doctor. I told my employer that I would return part time on 18th October 2015. My employer informed me that they cannot accommodate me and that when I am well to return on a full time basis. I informed my manager by e-mail that my doctor has advised I return to work on November 30, 2015. On November 18, 2015, I received a termination letter from my employer informing me that I had overstayed my medical leave. There have been other employees that had medical problems and were off work for 24 weeks. I was gone for 21 weeks. Do I have a case of discrimination if I was replaced by a younger worker and at a reduced salary?