Many perpetrators use drugs to render someone unconscious or unable to give consent. Whether you knew you took the drugs or not, it is never your fault.
If you are concerned about having been drugged, you can ask that a toxicology kit be completed at a hospital emergency room. This kit may be able to detect rape drugs if it is done within 72 hours of the assault. You can have the toxicology kit performed without reporting the assault to the police, and you can receive the results of an unreported toxicology kit anonymously.