I have sometimes noticed that in the shower after a run, I smell like ammonia.
I just got back from a short 2 mile run (after five days off, I know, I know) and the smell is particularly noticeable, so I googled it. This is what I found:

The reason this happens is ammonia is a by-product of protein metabolism. When you run hard and long, your body burns protein and carbohydrates for fuel. When the carbs are burned off (or when there aren’t adequate carbohydrates), the distinctive ammonia smell presents itself.

The fact that ammonia is produced on every run doesn’t mean the smell is noticeable afterward. Usually the smell is only present when there is the carb deficit. This increases the ammonia levels in your muscles and the ammonia is then picked up by your blood and carried to your respiratory system. When this occurs, you can usually smell it. In addition, some of the ammonia will show up in your sweat which makes the odor even more noticeable.

The smell usually isn’t anything to worry about. But if it persists, you probably aren’t taking in enough carbohydrates. If you emphasize carbohydrates before, during and after running and the ammonia smell is still around, you should get checked out by a physician.

Some other dude in the comments mentioned that it's just because we're getting older, but CLEARLY that cannot be the case. I am a spring chicken! Really. No, really.

Although last night I had a salad for dinner with some jalapeno ranch dressing, and a couple blocks of peppermint bark, and this morning I had a sugar free vanilla latte (ok no carbs there, hmm.), I haven't eaten yet today and normally I don't eat before my morning runs. Maybe I should change that.