Like Sailor said sourness happens in cheese that is still too young. Some other signs of over-acidifying are chalky, hard and brittle cheese and cheese that cannot melt. These don't have to be apparent in over-acidified cheese, but if they are apparent - then your cheese surely is over acidified.
If your texture and acidity are perfect and you have aged your cheese long enough and properly than this may be an issue of under-salting.