At my first dojo the head instructor was the only Sensei, and I certainly was not him.
At my current dojo, its acceptable to address any Yudansha as Sensei.
Ask what is the etiquette in your dojo/organization.
Always be respectful and accept respect (even if you are uncomfortable with it).
I had a similar experience. At my current dojo, anyone who teaches is Sensei. At my first dojo, only the head instructor.
I thank whatever benevolent spirit kept me from correcting the first student who called me Sensei, but being addressed that way still makes me twitch.