What can cause a heart murmur to suddenly get worse?

THE HEART VALVE: Creating the murmur has become thicker or less mobile or more floppy.

Heart valve,disease: I agree with dr villaplana that one of the concerning causes is the "doors" (valves) between the "rooms" (chambers) of the heart have changed (gotten thicker or weaker) or have been damaged or even infected. In some of these cases, symptoms would become severe, such as chest pain, dyspnea. Other causes are anemia, dehydration, exercise, other diseases. Good exam and echo can help find cause.