Employment Law Questions? Ask an Employment Lawyer.
You need to file a complaint with the Department of Labor. That is the entity that investigates and ultimately enforces FMLA infractions against employers.
That entity can investigate for free. Furthermore, it is the only way to preserve your rights to sue and to obtain a right to sue letter for this type of infraction. Going through their process is a legal prerequisite to filing a lawsuit.
If you are presently being retaliated against, then it's time to just go straight to that entity now rather than trying to work through HR one more time. Filing the complaint is a legally protected activity, meaning that further retaliation for filing would be illegal, so this will get you some protection from retaliation.