Why does a person get light headed or dizzy after blowing up balloon or blow horn?

Breathing will release carbon dioxide while catching oxygen from the air. When you breathe out, you release carbon dioxide from your lungs. This will happens faster when blowing horn or ballon, makes the carbon dioxide level very low.

Carbon dioxide should be dumped, but the concentration should be kept at certain levels because CO2 help maintains blood pH. Carbon dioxide makes the blood more acid, so losing many CO2 will result in the blood become more base. This pH change(respiratory alkalosis) lead to vasoconstriction of the blood vessel that can cause various symptoms like light headed or dizziness.