The medical insurance provided by your employer likely qualifies as a Welfare Benefit Plan so you are protected from any retaliation based on the expenses your health condition is causing to the Plan. Violation of the FMLA and disability discrimination laws may also be implicated in your situation. You would need to speak to a lawyer in order to flesh out all of the necessary facts for a proper legal analysis of your claims.
Our firm is licensed to practice law in the State of Wisconsin and offers information on federal laws that may apply to residents of other states, however, we offer no advice as to the state laws outside of Wisconsin.
The combination of facts you have provided strongly suggests that your employer may be discriminating against you for taking medical leave and/or the use of your employer-sponsored insurance. That would invoke protections (and create claims under) FMLA for the medical leave and ERISA for the use of your insurance. I would strongly recommend that you speak with an employment attorney in your area who can investigate your situation and discuss your options.