Code 44 O2 sensor, or bad CAT?

I just finished some repairs to revive a non-running 94 S-10 Blazer (Vortec). I drove it around the neighborhood for a couple of days with no problems. Yesterday I had a muffler and exhaust pipe installed. Now it shows a code 44. It was running rough after replacing the exhaust parts and I wonder; is it the O2 sensor, or is this just a symptom from the extra pressure and possibly just a bad CAT? If it is the sensor, could anyone tell me which one is the 44? there is also another O2 sensor(code 45) and I want to make sure I pull the right one if this is the problem. Thanks!

Update: engine runs until it gets hot, then rough idle, then shuts off and won't start again until it cools down.

According to the book, the code 44 is the "left bank". Is the left bank the driver side or passenger side?

Exhaust doesn't seem restricted, so is it safe to rule out the CAT now?
Thanks.