Is expiation (kaffarah) obligatory for one who intended to fast a make-up (qada’) day but then menstruated on a “day of doubt” (yawm ash-shakk), and was unable to make up what was due before Ramadan?

You must make up for this day after Ramadan ends. There is a difference of opinion regarding the obligation of expiation (fidyah) upon you. The Maliki school of thought holds that it is not obligatory for you, but if you offer it as a precaution, that would be good.