What makes you think that:
1. Your job responsibilities can't change? Even with FMLA all they have to do is provide a substantially similar duties, not necessarily the same. As you were pointed out above, this isn't even covered by FMLA most likely. The employer is free to change your responsibilities at any time.
2. Employers can't fire you for complaining? There are very limited times when an employee can make a complaint and be protected. This doesn't sound like one of them.
Just because you don't think it's fair, doesn't make it wrongful termination. As pointed out, file for unemployment and see where that goes, meanwhile look for another job.

