Stayed late to cover a call-off Monday, hours got cut today.

So I work evenings and nights, and Sunday night I got a call telling me the Monday morning supervisor called off sick, and asking me to stay a few hours until somebody could come in and take over. I told my boss sure, I'm happy to earn some overtime pay. Today, I get a text message telling me to come in late today and tomorrow because corporate told him to “make some adjustments” to avoid overtime. Guess who won't be staying late again in the future!