The Family Medical Leave Act only applies to full time employees that have worked for that company for over 12 months. If you meet those conditions and you requested leave under that act, you are guaranteed employement when you return. If you do not, the employer can fire you whenever they so choose.
This doesn't mean that the employer can't fire you a day later for another reason.
The legal consequences of termination vary from state to state, so the best thing to do is contact an employment lawyer in your state and find out if the termination was legal or not.

Thanks for the info!