That definitely sounds like auto iso behavior. When auto iso is on, you are setting the base iso. It won't go lower than what you set but it will go higher. Your shutter behavior is defined in the auto iso menu too. You can set a minimum shutter speed which means the camera will try not to go below that shutter speed. It gives preference to raising the iso before going below the minimum shutter speed. The other choice is auto minimum shutter speed. That picks a low end shutter speed based on focal length and again gives preference to raising iso before lowering shutter speed.