NCO Financial Systems Sent me a bill for apparently nothing; what did I pay for? Horsham, Pennsylvania

I'm a young adult who is stuck in the perpetual strife that is the whirlpool of student loan bills that I pay each month after leaving college, just like to many other former students in the country. So, recently I'd moved halfway across the US and had a slight lapse in jobs for a couple of months while I searched around; typical for a long move such as that for a man my age, I figure.

Unfortunately, though, in that time I'd accrued a few months' worth of bills from one of my student loan companies. Which was to be expected, and of course I was well aware of the fact that I'd have to catch myself up once I got a job - which I now have to this present day. As I'm catching myself up though, I receive a bill from NCO Financial Systems. Now this might be foolish on my part for not putting it in my record and/or simply a lapse in my memory, but I cannot remember if it said what the bill was for. 

In my rush, I'd naturally written a check and sent it out, assuming that it was simply that my student loan company had contacted them and now NCO was collecting the debt instead (should have done some research). Eventually I kept getting bills from my original student loan company (of which I am currently paying off normally again), so that's good, but now I'm wondering just where in the world the money from the check I sent to NCO went to? It certainly didn't go towards ANY bills that I pay. I might as well have thrown that money in the garbage.

I'm quite unaware of what I should do, if I CAN do anything. Any sort of aid would be greatly appreciated.