I've noticed that when a pokemon has an ailment or is low on health, it's voice changes. I'm not sure if it's true, but if someone could confirm this, then i don't have to go crazy thinking this.

Yeah, this is normal; its voice is slower and more drawn out, like it's groaning in pain. After all, it is logical for a Pokemon with low HP to not sound their cry as fast, due to the beating they've had to endure.

Give them a bit of a break. (Just my little joke. It sucks, doesn't it.)
- DNA