Are debt collectors allowed to charge interest?

Hello! I have a question that has been bugging me for some time. I was living in Las Vegas back in 2009-2010. I was living in an apartment complex called Positano apartments. I moved in November 2009 and had to break my lease May 1, 2010. One of the room mates moved out, I couldn`t find another, I was on unemployment, and couldn`t find a job so I had to pack up and go to Jersey to live with my parents, like most people had to. Well I gave the apartment complex some what of a notice since my roommate just up and left one day. Well the bill I got back was outrageous lets just say over $8,000.00 and being that I was on unemployment and was trying to find a job in New Jersey I couldn`t afford such a thing. Now I am married and stationed in Germany with my husband. We are trying to pay back some of my debt little by little. The bill is now with a collection agency called Hunter Warfield in Tampa, Florida. Now is it right for them to be charging me interest???? I never agreed on such terms. Everytime they send me a bill they add another $200 to my bill. Is this right? What should I do? Considering the rules above your site states:
???The debt collectors are not allowed to charge interest rates and fees that were not agreed upon by you or your creditor.
My sister tried calling yesterday and they were very rude to her. Going as far as saying "How do you pay your other bills then?" and asking for somebody else they can contact her through which in fact on your site you also state:
???FDCPA restricts debt collectors from contacting a third party who does not owe money to the creditors. Third party may include employers, neighbors and relatives of the debtor. However, a co-applicant of a debtor can be contacted.
Is this right and if not is it something I can report?