My boss is putting too much work on me. Now he's threatening to write me up if I can't keep up. If that doesn't work to let me. Do I got a case.

On those facts, it doesn’t sound like there’s a legal cause of action.  Florida is what’s called a ‘right to work state.’  It is a bit of a misnomer, but essentially allows an employer to fire an employee for any reason or no reason at all.  There are certain exceptions for race, religion, gender, etc., but under the facts you’ve given it doesn’t sound like an exception is applicable.  Of course, if you have an employment contract that limits the scope of your employment, it is possible there’s a breach of that contract by your employer.  Barring some additional facts that would change the situation, I don’t see a cause of action.  That said, keep in mind that facts will dictate so you would probably find it helpful to speak with an attorney regarding the specifics of your situation.