Instead of going to your Chevy dealer, I'd go practically anyplace else with this problem. Why? Because your problem is almost certainly associated with the new muffler you put in, which means it is no longer a warranty issue, and it's only going to upset them.
First place to go would be your local Autozone or similar parts store which will have a scan tool that can read your CEL, tell you the code, which you can look up online. Very likely it's an o2 sensor, but since the dealership didn't tell you the code and gave you an answer that makes no sense, it is at least possible that there's something else going on and the muffler change is just a coincidence.
Next place to go is the place that did the install. New muffler isn't a GM issue, it is possible, however, that it is something that the install shop did wrong.
Likely answer though is that, depending on what you actually had done, this is a permanent recurring problem because your car is tuned wrong for your muffler setup. In order to get any real benefit out of a new exhaust (other than the change in sound) you need a new tune anyway. The latest version of the Trifecta tune not only allows you to get the power out that you've added, but it also includes the ability to clear CELs.
Finally, if you're monkeying around with your car in any substantial way, I'd recommend getting an UltraGauge or something similar to it, this plugs into your OBDII slot and gives you a constant readout of many many details of your car, it also includes the ability to check not only your CEL's but any "pending" CELs, which again you can check on the internet, and can clear at a whim.