Before I ask my question I want to give a little.

Before I ask my question I want to give a little background that I feel could help diagnose this problem I'm having. I have a 06' Cobalt SS Supercharged. I do have an aftermarket muffler on it. But everything else exhaust wise is stock. The other day I was driving and my car went from being loud to basically factory quiet. It did not sound like I had an aftermarket exhaust AT ALL. Also, when I accelerated to kick my supercharged in I notice I loud "whistle" or "whinning" sound that did not sound like my supercharger (I know what that sounds like). I noticed when I parked an odd oder coming from my car that I did not smell before. It wasn't overbearing or any smoke, but enough to notice. I tried starting it yesterday and it did not start. I started it today and fired right up, but same quiet exhaust. I came to the conclusion that my exhaust could be clogged or my catalytic converter is bad. What could be causing this issue? Also, I ran a diagnostic and I got a U2100 error code. I disconnected my battery to reset my ECU with no avail. Please give me some incite on what this could be. I don't want to spend a lot of money if I don'y have to. Thanks in advance.