Florida is an "at-will" State which means a person can be terminated for almost any or no reason. As long as the termination isn't based on the person's religion, race, gender, and so on, there is very little recourse for a termination. There is no legal avenue you could pursue in this case. The only thing you can really do is try to convince the employer that the medicine from the hospital caused a false positive. You may wish to have your doctor write a letter to that effect.
This is not to be considered legal advice nor does an attorney-client relationship exist.
If you want your job back and the company is saying you can get your job back you need to prove you do not take illegal drugs. Go to your own doctor and request a full lab work up and check the box for all of the illegal drugs (if in fact you really do not imbibe). Have your doctor who has been prescribing you all of the narcotics, pain meds, etc. and is also aware of the drugs that went into your system through anesthesia (ask him to use Dr.'s medical notes that he can rely on and reference in his letter and attach as an exhibit) write a complete letter outlining the same and confirming why a drug test would have tested positive for the amphetemines. Match the dates up prior to requesting the letter so your doctor can explain away the residual positive. Bring in clean drug test, or dirty only with the meds doctor is prescribing, and proof of everything we have discussed.
Unfortunately, if you are in an "at will state" my counsel's proffer above still holds true. Good Luck.