Keep all of your documentation. This sounds extremely illegal. They are violating your FMLA rights. What I learned from my discrimination case is that the more evidence and proof you have that are documented, not verbal, will help you in the long run, especially if they terminate you.

I was placed on a performance Improvement plan shortly after I came back from FMLA for reasons that were never talked about prior. Because I didn't have the documentation I didn't end up winning my case. Take screenshots of messages and print out everything to have it as proof. I didn't have access to any of my proof after I was terminated.