Is my student loan collection agency ripping me off?

After 4 years of college, I aculated a student loan debt of about $17,000 ($19,000 with interest) which I had consolidated with Nelnet. I struggled to make the first 3 payments, but eventually stopped after getting fired from my $8/hour job. Two and a half years later, I received a letter from a company called "Windham Professionals" stating that they purchased my loan from Nelnet and that I now owed them a whopping $45,000 with no explanation as to how they arrived at that number other than to say that $7,000 was for 'collection costs'. Does this sound right? It took 4 years to aculate the original $17,000 debt and a little over 2 years to nearly TRIPLE that number? What's keeping them from charging me $1 million? I have since made 14 on-time payments to Windham to rehabilitate the loan, but I would like for that $45,000 bill to at least be cut in half. How do I do this?