Why do I smell ammonia?

After a run or other workout, I will smell the faint odor of ammonia when I walk into my house. My face is usually damp with perspiration, and so I guess that maybe there’s some reaction with whatever’s in the air and my sweat.

I don’t smell ammonia when I walk into the house any other time, even after hours or days away. And it doesn’t matter what time of day or weather conditions.