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Unfortunately, Texas is what is known as an "employment at will state" which means an employer can do pretty much as thy see fit so long as it isn't motivated by race, age, gender, etc.
Texas doesn't have ny laws restricting how long a private employer can go back in time to sanction an employee or what type of thing they can look at in deciding a sanction.
In addition, what makes it even harder is that your employer is just following the directions of a client and that provides another level of protection for them.
Many people believe that they have more rights as an employee than they do, Texas is one of the strongest pro-employer states that there is, so I'm going to put a link here to the TWC website that explains a little about employment at will in Texas.
You can consider hiring an attorney to write the company a letter advising them that releasing any information about your drug test is a violation of HIPAA as well as informing them that a failure to take a drug test is not the same as failing one. However, those are just threats, a private citizen can't sue someone for violating HIPAA.