What's wrong with the cheese?

The cheese I bought recently seems a little different.  I like to add cheese (mozzirilla or cheddar) to my soup. In the past, the cheese would melt well into the soup.  However, for the last two times, the cheese doesn't seem t melt and it tastes kind of rubbery.  

Is it because the quality of cheese is not so good?  Anybody knows?  I looked at the bag, the cheese is made of pasterized milk.