Employment Law Questions? Ask an Employment Lawyer.
You should explain to your employer. Really, this depends on your relationship with your employer. Unless you have a contract then you have no employment rights in Texas. Texas is an "at will" state. This is true of most southern states. You can be fired for any "non-discriminatory" reason. I would take a copy of he doctor's findings and explain what hapended. If the facts match, then it's likely that your employer will overlook the positive test.
I am not familiar with DISA. Texas law is he problem. There are no protection in Texas, unless you are a contract or public employee. Then you have to follow the adminstrative process, which rarely ends up well.
Please rate and conclude question, unless you have more questions.