Having a booster for MMR and wondering does it hurt?

Not as much. Doesn't hurt as much as getting one of the illnesses. The united kingdom is having to witness what happens when enough people back away from a vaccine. Britain faces a measles epidemic (as ireland before it) & has been enduring mumps as well with serious--occasional grave- consequences. The MMR shot--stings for an instant when given in the soft tissue of the upper arm but saves lives. Maybe yours.
MMR. The MMR vaccine is no more painful than any of the other vaccines you are required to receive.