Rpm to drop

Your AC compressor may have kicked in. It should not be more than a slight RPM drop, may be 50-100 RPMs at most.

I didn't have my AC on nor the defroster. I read it could be a spark plug problem?

I would look elsewhere. If it was a plug issue, you would also expect to have a distinct miss or stumble, not just a drop in RPM's. Sounds more like a sensor in the engine controls. Have you checked to see if it is throwing any codes?

Cylinder 2 miss fire code