Collection Agency Interest Hike!

I attended a college Mercy College in NYC for one semester and requested financial aide to pay for this one Semester. The tuition was $7,000 I paid $1,000 in the beginning and continue to take my classes however I later found out that they denied my financial aid and I was stuck with a $6,000 bill to pay. However, I was seeking employment but couldn't find a job so a collect agency sent me a letter stating that I have to pay and my new balance was over $7,000 in 2010 so I sent in a payment this year after landing a great job and now I checked my credit to start cleaning things up and my balance is over $9,000 and I am trying to figure out how and why is jumped up so much with in two years. I haven't gotten a letter or anything.