Why does it hurt do breathe the C02 already in your lungs but not helium?

People breathe in helium as a painless way to commit suicide. I know from experience that breathing in helium from a balloon feels like normal air. But when I breathe in the C02 in my lungs, (like breath in and breath out the same, recycled air) it starts to hurt, and I feel the need to breathe. Why does that happen with C02 but not helium? Do you understand what I'm saying?