Ammonia smell in nose after working out

Not to scare you.

But this could be a couple things. If your sweat smells of ammonia this could be natural: read here http://health.howstuffworks.com/skin-care/information/nutrition/ammonia-sweat-smell.htm

OR.

This could be a sign of brain tumor. Usually you would smell this at other times too! But you can get it only when doing aerobic activity. Not uncommon.

I would bet on the former.