All the medical marijuana card does for the holder is to perhaps provide a defense when charged with a criminal law violation by the state and to allow the person to grow and possess marijuana within the limit set by the government.
Having a card does not prevent an employer from firing the holder.
If an employer does not want its employees to be smoking marijuana (or ingest marijuana in some other way), drinking alcohol, or taking any other substance, the employer can have a policy to fire those employees who violate the policy as long as the employer is not violating any law against unlawful discrimination.
If you are in some sort of union, you can check whether your union would back you up.
There're a couple of issues here: (1) the reason for having the medical marijuana authorization in the first place, and (2) the DLI claim.
Depending on the facts of either, if you were fired from your job or demoted or whatever, you may have a colorable claim for disability discrimination and/or retaliation in violation of public policy (e.g. employer not really digging the DLI claim).
Where you can establish an initial showing that you were discriminated against or retaliated against, the burden then shifts to the employer to show that the basis for your termination was legitimate (the employer probably has a legitimate concern about employees smoking marijuana). Were that the case, the burden then shifts back to you to show that the termination was a pretext - a false front - for discrimination and retaliation; e.g. I have had this authorization for years and years, and my employer has always known about it and never disciplined me for it, BEFORE my DLI claim.
You have to start from the basic premise that WA has an 'at will' employment policy as a state. That means that in the absence of a contract (which can be explicit or implicit) or CBA, you do not have an expectation of continued employment and you can be fired for any reason, right wrong or otherwise, so long as that reason is not discriminatory (as dictated by the Washington Law Against Discrimination) or against Public Policy (subject to relatively new case law limiting that; e.g. Cudney & Korslund).
It's probably worth a consultation with an employment law attorney, to know your rights, but without knowing more about your situation, it would be challenging to give you a more certain analysis.