Hard cheeses are difficult to infect -- especially long aged ones like cheddar. There are some moulds that produce ammonia that live on cheese and it's a good thing too, because we rely on it for cheeses like Camembert and blue cheeses. There is a thread on here somewhere where linuxboy explains how there are no pathenogenic moulds that can survive on hard cheeses.
On thing to note: ammonia is a really strong base. If there was a lot of ammonia in the cheese and it was mixed with coleslaw (acidic) the ammonia would react with the acid and produce salts -- and would no longer smell like ammonia. Secondly, if there was enough ammonia to make the cheese smell strongly of ammonia, then the pH of the cheese would be up around 7 and the cheese would liquify (become all gooey) -- like a well aged camembert (Fun trick -- mix cheese with baking powder to make a cheese spread... search for videos on Youtube if you don't want to do it yourself). So the odor you are detecting is almost certainly not ammonia. Probably it's just a funky cheddar. But remember the point at the top: there are no pathenogenic moulds that can survive on cheese.
In fact, I think I know exactly the smell you are smelling and it's actually because the cheese in your bag is *not* cheddar, but a cheddar/gouda/swiss mix -- and you aren't used to the odour of the other cheeses. In Japan, where I live, they sell (pizza cheese) which is pre-grated, but contains whatever cheap cheese they can get on the international market.
2 lessons to be learned: 1) Don't fear the cheese. 2) Don't buy pre-grated cheese :-) (OK, I still do because without me making it, it's the only way I can buy non-processed cheese :-P)
