Thank you for your question. I look forward to working with you to provide you the information you are seeking for educational purposes only.
The employer cannot cancel your insurance without notice while you are on FMLA or upon your return from FMLA. This would be a violation of both COBRA (for not notifying you and offering COBRA benefits) and FMLA. For starters, and people have tremendous success with this, you need to file a complaint with the US Department of Labor
for violation of both COBRA and FMLA and they will work to try to resolve the issue and the employer would be liable for the medical costs that should have been covered by insurance.
If the US Department of Labor cannot resolve the complaint, then you have grounds to sue the employer for improperly canceling your insurance contrary to FMLA and COBRA and seek damages from them including the costs of any uncovered medical care.
Also, you have grounds, if they terminated you in retaliation, to add a cause of action in your suit against them for the retaliatory discharge.
In addition to the above, you also can pursue workers compensation
claim if the cause of your subsequent illness through EDD or at least apply for short term disability
for the time you cannot work.
You really need to consider getting a local employment law attorney to represent you because the stress of doing all of the above can make you sicker if you are already still not fully recovered.