I was hired as an assistant manager for a seasonal store then was demoted for "hear say" comments.

I just want to know if I have to the right to call the company and complain about my treatment there. After I was told I was being demoted the I asked the store manager why and he said "because I was told you called this job a joke and that you think you are better than everyone else because you are a college graduate". This was all based on the comments of an employee that has schizophrenia and is only re-hired every year because they feel bad for him. I knew the store manager didn't like me and would have fired me if he could but their wasn't a cause. I was never told that I had done any of the duties of being an assistant manager wrong to deserve the demotion. I was asked to stay on as a cashier but I ended up just quitting.